This new genus is required for a group of three shells from deep water in the Gulf of Oman, the Philippines and Japan. These shells are very small, 6-10 mm. (0,25-0,375 of an inch) in height, yet judged by the number of whorls, 8-8,5, they are apparently not juvenile and it is doubtful if they will be found to attain a much larger adult size.
The shell is Cochlespira-like, but very thin, is strongly carinated to coronated at the periphery, and is devoid of, or almost devoid of spiral sculp-ture. The chief diagnostic character of the new genus is, however, the form of the sinus, which is quite unlike that of any other cochlespirinid genus, and bears more resemblance to that of the daphnellids, in that it arises tangentially from the suture, after which it is quickly produced far forward in a great arcuate curve. In the typical cochlespirinid shell, where there is no spiral sculptural impedement, the sinus is deep U-shaped, its apex more or less median on the shoulder slope. The presence of an intermediate lamella upon the shoulder area, as in Cochlespira, results in that sinus being restricted to the area between the suture and the lamella, but it is still a U-shaped sinus. In Thatcheria-syrinx, however, there is no apparent physical cause why the sinus should be so narrowly sutural.
The genus owes its name to its superficial re-semblance to a miniature Thatcheria.