INVBETE BEATE PA L.E( )NT( )LOGY. 24 3 



the reverse. In regard to this hitler character, however, that usually ver\ 

 accurate naturalist was. doubtless from inadvertence, certainly mistaken ; 

 the arrangement of these parts with relation to the right and left valves 

 being exactly as in Corbula proper. 



In regard to the propriety of retaining Mr. Conrad's section Aniso- 

 rhynchus, as subgenerically distinct from Corbula, there seem to me to be 

 even stronger reasons for doubts than in the case of the South-American 

 type for which the name Pachijodon has been proposed, Mr. Conrad pro- 

 posed this name (Anisorhynchus) for the reception of a species described by 

 me under the name Corbula pyriformis* from some brackish, or nearly fresh 

 water deposits in Wyoming. I believe he proposed to make the separation 

 mainly, if not entirely, on the ground of the apparently brackish-water habits 

 of this species, without knowing the characters of its hinge and interior. I 

 have, however, had an opportunity to examine good specimens, showing the 

 hinge and interior of both valves perfectly; and the only differences I could 

 see between this shell and the typical forms of Corbula, are, that its hinge- 

 tooth is furrowed, and its valves and beaks more nearly equal, and the latter 

 more incurved than is usual in that genus; while the shell presents a Ne&ra- 

 like form, being narrowed or rostrate behind. It is worthy of note, however, 

 that Mr. Grabb has described a marine Cretaceous species of Corbula in vol. 

 II of the California Palaeontology, presenting almost exactly the same form 

 and general appearance.! In addition to this, another species found directly 

 associated with the C. pyriformis, in Wyoming, and named C. Engehnanni 

 by me, has not this Neaera-Xike form, but agrees in general outline with 

 many marine types of Corbula 



That the types of both Pachyodon and Anisorhynchus may have' pre- 

 sented some generic differences in the anatomy of the animal from Corbula. 

 is admitted ; but, in the present state of our knowledge of these shells, it 

 does not seem to me desirable to separate them more than subgenerically.} 



The existing South-American brackish-water group Azara, d'Orbigny, 

 is also included, by several eminent authorities on conchology, as a subgenus 



"See Mr. King's Report U. S. Geol. Survey of the Fortieth Parallel, part- Paleontology, pi. 17, 

 figs. 2, a, b. c, d. 



t Corbula alce/ormis, Gabb, Palseont. Cal., II, pi. xxis, fig. 63. 



; Dr. Stoliczka mentions brackish-water Indian species that cannot be distinguished from typical 

 marine forms of Corbula, excepting that their shells are thin and resemble Necera in outline (PaliBOnt. 

 Iudica, III, 35). 



