Shell patelliform, not nacreous, symmetrical with an entire non-sinuated margin and a posteriorly inclined apex with a (usually deciduous) spiral nucleus; muscular impression horseshoe-shaped, interrupted over the head.
Animal with a prominent head and muzzle, the males with an intromittent organ at the base of the right tentacle; a single lamellose asymmetrical gill (resembling in form and place of attachment the gill in Acmaea) between the under surface of the mantle and the upper surface of the body from a point above and behind the head, extending around toward the right, and even backward on the right side; attached only at its base. Eyes wanting in the known species. Anus anterior, opening in a papilla above and behind the head. Mantle margin and sides of foot plain, without epipodial papillae or processes, but they are sometimes present behind. Radula with a small or moderate hardly raised rhachidian tooth (the cusp in one species obsolete), three moderate inner laterals with denticulate cusps, a larger denticulate major lateral with a stout and twisted stalk, and on each side a stout base from which spring numerous slender uncini hooked at their tips. There is no jaw. The dentition resembles in a general way that of Parmophorus and of some species of Helicina.

Shell up to 12 mm long, strongly depressed to strongly arched, anterior end occupying 50-84% of shell length. Protoconch reticulately sculptured. Teleoconch sculptured with fine, smooth, radial threads. Periostracum thin and smooth. Central tooth present or obsolete, radula otherwise typical of family. Cephalic tentacles similar. Copulatory organ large, grooved along posterior edge, situated on side of foot immediately below tip of right oral lappet. Two epipodial tentacles at posterior end. 'Eyes' present or absent.
Remarks. Despite extensive enquiries I have been unable to ascertain the whereabouts of the holo¬type of Cocculina rathbuni, supposedly from U. S. Fish Commission station 937 (off Martha's Vineyard, 925 m), the first listed after the original description. However, an original syntype ('Blake' station 195, off Martinique, 919 m, NMNH 126807) and a specimen recorded subsequently by Dall (1889) ('Blake' station 230, off St. Vincent, 849 m, NMNH 95122) conform perfectly with the description (Dall 1882) and illustrations (Dall 1889, pl. 25, fig. 5, 7, 7a), so I accept them as authentic representatives of the species. Distinctive features are the very large size (length of St. Vincent specimen 12 mm; a specimen 33 mm long was reported by Dall, 1882), subcentral apex, thin strongly depressed shell, thin smooth periostracum, and the teleoconch sculpture of fine, crisp, smooth, wavy radial threads. The surface of the protoconch is etched in both specimens, but fortunately the St. Vincent specimen retains some of the outer shell layer against the suture, which has reticulate sculpture as in Coccopigya.
In his descriptions of the animal of C rathbuni Dall (1882, 1889) made no mention of a copulatory structure, even when recording the existence of one in Coccopigya, and concluded that his specimens
were females. After the manuscript for this paper had been submitted for publication, a specimen agreeing closely with C rathbuni was obtained living by Dr P. S. Mikkelson (Indian River Coastal Zone Museum, Florida) at 124 m off Fort Pierce, Florida, and kindly forwarded by Dr J. H. McLean. The external animal morphology of this specimen is closely similar to that of C. cervae Fleming, but its radula differs in having well developed central teeth. With our present limited state of knowledge of the Cocculinidae, it seems best to place C cervae in Cocculina despite its much more strongly posterior shell apex, and obsolete central teeth. Degree of development of central teeth may be no more than a species character in this instance, since central teeth may be present or absent in dif¬ferent parts of the radula of at least one species of cocculinid (Hickman 1983, Fig. 15).
A species very like C cervae occurs in the Lower Miocene of New Zealand (see below), while another is to be described from Prince William Sound, Alaska (J. H. McLean in press). The anatomy of C. laevis Thiele (1903A), a species very close to C. rathbuni in shell morphology, has been described in considerable detail by Thiele (1903B).