Elongate, turreted, with about 15 inflated whorls; upper whorls sculptured with three spiral cords and numerous fine axial striae, later whorls with four major and four minor spiral cords, the central one carinate, crossed by 12-13 weak to strong axial ribs forming a cancellate pattern, intersections sometimes beaded, body whorl strongly constricted, with broad, flattened cords; aperture ovately elongate, with a flaring outer lip. Pinkish tan to light brown, with dark brown spiral lines and bands, blotched with white, especially on the beads. 1.9 cm.

Shell narrowly turreted, elongated, comprising about 15 inflated whorls. Protoconch small, comprising one smooth whorl Upper teleoconch whorls sculptured with 3 spiral cords and numerous line axial striae. Adult teleoconch whorls sculptured with about 4 major spiral cords and 4 minor spiral cords; central cord frequently large, carinate; subsutural cord with minute axial pleats Spiral cords crossed In 12-13 weak to strong axial ribs, forming cancellate sculpture intersections commonly beaded. Suture moderately impressed, distinct. Body whorl strongly constricted, elongated, sculptured with broad, flattened spiral cords and weak axial striae and incised lines Aperture ovately elongate with moderately elongate, slightly reflexed canal, concave columella, flaring outer lip of aperture, thick and .smooth at edge. Shell color pinkish tan to light brown, with dark brown spiral lines and bands, and blotched with white, especially on beads
Radular ribbon small, short about one-seventh the shell length rachidian tooth with square basal plate having slight median posterior projection and weak lateral told at each posterior base; cutting edge with median, spade-shaped main cusp flanked on each side by 2-3 pointed denticles. Lateral tooth with rhomboid basal plate having short lateral extension and wide central pillar with small pustule, cutting edge with large pointed main cusp, one inside denticle and 2-3 outside pointed denticles. Marginal teeth spatulate having wide bases and curved tips with long pointed main cusp, 2-3 inner pointed denticles and 2 outer denticles; outer marginal tooth same but lacking outer denticles.

The type lot of Cerithium macrostoma Hinds originally consisted of five specimens glued to boards The one surviving specimen is labeled "probable holotype”. It is the figured specimen in Sowerby’s Thesaurus Conchyliorurn) and in Conchologia Iconica (pl 17, figs. 118 a.b) The type of Colina gracilis H. Adams has not been found. Considering that the description is ambiguous and no figure was presented, it seems prudent to regard this taxon as a nomen dubium. H. Adams (1866) noted its close resemblance to Colina macrostoma and later remarked (1866:308) “that the species I lately described as C. gracilis I find has been since described by Mr. G. B. Sowerby under the name of C. coarctata.” Although the type of Cerithium coarctatum has not been found, Sowerby’s figures adequately represent it. and indicate close resemblance to Colina macrostoma. The many synonyms of this species have resulted from the failure of authors to appreciate the wide range of intraspecific shell characters Recognition of the variability of Colina macrostoma was first noted by E. A. Smith (1884:66-67), who suggested that Colina costata, C costiferum, C. pygmaea and C pupiformis were all “. . . mere variations of one and the same shell and who noted the variation in whorl number and shape-due to loss of the upper spire." While it may be difficult to believe that extreme phenotypes are conspecific, examination of many museum specimens and the types of the nominal species listed in the above synonymy reconfirms E. A. Smith's conclusion.
Colina selecta Melvill & Standen, 1898 is conchologically very close to Colina macrostoma, and may be a synonym of this species, but this remains unresolved, due to lack of comparative material.
So this species is highly variable in shell shape and sculpture. Older adult shells have a truncated, pupate appearance due to decollation of early whorls and erosion of the apex, and these look very different from non-decollate shells. Some specimens lose more of the early whorls than do others. When the apex of the spire has been broken off, the animal closes the opening with a spiral plug and frequently this is inset deeply within the old whorl.
There is a great intraspecific difference in whorl number and in the number and strength of the axial ribs. A range of phenotypes, from those having extremely slender shells to those with highly inflated midwhorls with wide axial ribs on their shells, may occur within the same population. Shell sculpture is highly variable but generally comprises 4-6 spiral cords of which the dominant one frequently forms a keel on the middle of the whorl. Spiral cords are crossed by 12-14 strong axial ribs presenting an overall cancellate appearance. Some phenotypes have small nodes at the intersections. The subsutural spiral cord has many minute axial pleats. The bodywhorl cords are flat and weakly sculptured with about 10 incised spiral lines. There is seldom any axial sculpture on the body whorl except for the axial pleats beneath the suture. Shell color varies from light tan to pinkish-brown, the body whorl and especially its incised spiral lines, frequently being brown. These show through on the inside of the outer apertural lip

Indo-West Pacific; North West Cape, Western Australia to Mackay, Queensland.

This species appears to be confined to the Indo-West-Pacific, having a distribution from Japan south through the Philippines, Indonesia and tropical Australia, and into the Indian Ocean as far west as eastern India and Ceylon.
Shells from Japan and the Philippines are slender and highly elongate, while those from Australia and the Indian Ocean tend to be shorter, broader, and more pupate. These shorter phenotypes have been given the names Colina pupiformis and Colina pygmaea. Initially I was inclined to recognize these nominal species as a subspecies of Colina macrostoma because their shorter, more stocky shells tend to occur in more southern and western geographical regions. However, there are intergrades. And non-decollate specimens from these regions bridge the gap. Specimens from these parts of the range are frequently badly eroded and uncommon in collections, when more material is available for study, this putative geographic trend in morphology may become better documented. Currently, there is insufficient material to unequivocally resolve this issue, and it seems best to be conservative and regard these nominal taxa as phenotypes of Colina macrostoma.

There is little information about the specific micro-habitat of Colina macrostoma. In Japan and the Philippines, the habitat appears to be shallow, subtidal sea grass beds. Although spawn and larvae of Colina macrostoma are unknown, its protoconch morphology suggests a lecithotrophic type of development.