No. 89.J 69 



ATRYPA MODESTA. 



Palaeontology of New- York, Vol. i, pa. 141, pi. 33. 



This species, which was described under the name of j^trypa modesta (^Producta 

 modesta of Sat), was subsequently (Regents' Report of 1859, p. 66) left with 

 hesitation under the same designation, with a doubt expressed as to its generic 

 relations, and a suggestion that it might be related to LeptoC(ELIA. At the same 

 time, I suggested that Atrypa plicatula of the Clinton group might be of th© same 

 genus. 



Since that time, I have made careful examinations of some specimens of ^. 

 modesta; and though they are all solid, they nevertheless show, on cutting through 

 them or grinding down and polishing the surface, the existence of internal spires, 

 which appear to be arranged as in Ateypa. In regard to this fact, however, I have 

 still some hesitation, and farther examination is necessary to the full determination 

 of the generic relations of this species. 



Although having externally the form of Rhtnchonella, and the plicated sur- 

 face common to many species of this genus, it will be observed that the jltrypal 

 modesta has the sinus on the dorsal valve, and a mesial lobe or elevation on the 

 ventral valve. It likewise bears some resemblance in its beak and foramen to Tre- 

 MATOSPiRA, but there are still differences which are scarcely reconcilable with that 

 genus. 



Thus far, I have been quite unable to obtain separated valves of this species. 

 Solid specimens of the j^trypa plicatula, when cut through, have shown no in- 

 ternal appendages. My collection of these, however, is not large, and I have not 

 the means of making satisfactory examinations. The sinus of this species is in the 

 ventral valve, and the elevation on the dorsal valve is less like Rhtxchoxella 

 than it is like Trematospira or Rhtxchospira. These, and a few other of the 

 rhynchonelloid forms, are subjects worthy of farther investigation by those who 

 have large numbers of specimens of the species. 



Note. Being engaged in the study of this and some analogous forms, with a view to their 

 illustration in the next Report of the Regents upon the State Cabinet of Natural History, the 

 writer will be greatly obliged for any infoi-mation or specimens that may aid in the elucidation 

 of the structure of these fossils, and will be glad to give liberal returns of other fossils for any 

 specimens of separated valves, or those showing the interior structure. 



OBSERVATIONS 



Upon Orthis insignis of the Lower Helderberg group ( Pal. New-York, Vol. iii, pa. 173, pi. 

 13, f. 13 - 15) ; Orthis pyramidalis of the Niagara group ( Pal. New-York, Vol. ii, pa. 

 251, pi. 62, f. 2), and a similar species from Tennessee. 



An examination of the separated valves of Orthis insignis has shown that the 

 internal structure is so extremely different from that of Orthis, that the species 

 cannot with propriety remain under that generic designation. In the separated dorsal 

 valve there is a slender cardinal process, which, continued as a longitudinal septum, 

 extends from the beak towards the front of the shell where it is bifurcated, and, 

 expanding in width, reaches the inner margin of the ventral valve. The hinge-plates 

 or brachial plates are extremely thin and flattened, spreading laterally, and having 

 scarcely perceptible dental sockets : the inner margins are produced towards the 

 centre of the cavity, as if for the attachment of crura. In the ventral valve, I have 

 not been able to determine the form of the muscular impression. The angles of the 



