54 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



Mr. "Woodward, in Ms excellent Manual of the Mollusca (cited also by 

 Davidson), gives as the generic characters of Retzta : " Shell punc- 

 " tate, terehratula-shaped : beak truncated by a round foramen, rendered 

 " complete by a distinct deltidium ; hinge-area small, triangular, sharply 

 *' defined ; interior with diverging shelly spires." 



" Type, Terehratula adrieni, Yerneuil. Example, B. serpentina, Carb. 

 " limestone, Belgium, fig. 136." 



The species which have usually been referred without hesitation to this 

 genus, are of the type of B. serpentina. Recognizing the latter as typical 

 of the genus, I have published two species of congeneric form from the 

 C^irboniferous limestone of the Western States.* Certain other forms, which 

 have more nearly the character of Terehratula adrieni, I have united under 

 the Grenus Trematospira ; while others, with a difi"erent hinge-structure, 

 I have designated Rhynchospira. 



The species described under these various designations present wide dif- 

 ferences of character; and it now becomes very desirable to ascertain which, 

 if either, among them are congeneric with B. adrieni, which was originally 

 made the type of the genus. To begin with those usually referred to 

 Retzia, of the type of Betzia serjjentina, we have rotund, oval or ovate, 

 terebratulaeform shells, neatly defined in form, evenly plicated, without 

 mesial sinus or fold ; the valves nearly equally convex, evenly rounded, 

 and marked by numerous rounded costas, which are wider than the spaces 

 between. The hinge-line proper is short and nearly straight, one or both 

 valves having a little auriculate extension on either side of the beak, and 

 between these extends a sublinear or triangular area ; the dorsal side being 

 straight, while the limitation on the ventral valve is more or less arched 

 upwards. This area is always sharply defined, and is shown in the figure of 

 Mr. Woodward, while it is very differently represented in the figure of 

 B. serpentina given by Mr. Davidson. On either side, beyond this area, is 

 a smooth space on the margins of both valves ; but this gradually merges 

 into the plicated surface, and has no distinct limitation or important signi- 

 ficance. 



The area, in species of this type, is peculiar, and unlike the area in any 

 other palaeozoic trebratuliform brachiopod known to me. If one can imagine 

 the sharply limited area of some of the Spirifers contracted to the smallest 

 dimensions, with the margins still as sharply defined, he may have an idea 

 of its characters in the American species. The beak is incurved, and trun- 

 cated by a round foramen. The structure of the shell is punctate, and 

 internally it is provided with calcareous spires as in Athyris. 



In the specimens of this type which have fallen Under my observation, 

 there are no visible deltidial pieces under the beak. The dorsal valve, viewed 

 separately, has much the aspect of a small pectenoid shell, and is provided 

 interiorly with strong diverging lamella3, which extend beneath the cardinal 

 area of the ventral valve on either side of the centre ; while the same pro- 



* Retzia verneuili and R. vera. 



