GEOLOGICAL SURVEY OF THE TERRITORIES. 293 



Subsequently I received a note from Mr. Conrad, informing me that 

 he had proposed, in manuscript, a sub-genus, Anisorhynohus, for this spe- 

 cies. If this separation from Corbula is necessary, however, on account of 

 the habits of the species, I would think it should still stand as a sub-genus 

 under that group, or, in other words, that the name of the species should 

 be written Corbula (Anisorhynchus) pyriformis, because it seems to me 

 even doubtful whether Anisothyris is more than sub-generically distinct 

 from Corbula; while the species C. pyriformis is still more closely allied 

 to Corbula than Anisothyris is ; in fact, it agrees almost exactly, in its 

 hinge, muscular, and pallia! impressions, with Corbula, even in the very 

 few characters distinguishing Anisothyris from the same, and we have 

 almost nothing left but its Neara-like form, and brackish- water habits, 

 to separate it from Corbula ; while some marine species, such as C. alee- 

 formis of Gabb, present almost exactly the same form. 



It may be proper to remark in this connection that the South Ameri- 

 can group Anisothyris, or Pachydon, is much more nearly allied to Corbula 

 proper than was supposed by my friends Mr. Gabb and Mr. Conrad, 

 (although the latter placed it near Corbula) since they both thought it 

 had an external ligament, and no internal cartilage. On examining 

 good specimens of six of the South American species sent on by Pro- 

 fessor Orton to the Smithsonian Institution, however, I at once satis- 

 fied myself that these shells certainly had an internal cartilage in a pit 

 behind the tooth in the right valve, and attached to a process in the 

 left, exactly as in Corbula, excepting that the process is more oblique 

 and less flattened than is usual in that genus.* 



On mentioning these facts in regard to the presence of an internal 

 cartilage, in a letter to Mr. Conrad, he wrote back that a further exami- 

 nation had satisfied him that I was right in this, and that he now rests 

 the group entirely upon its sub-spiral beaks, a kind of rudimentary 

 posterior lateral tooth in the right valve, and a small accessory scar at 

 the upper end of that of the posterior adductor, together with the 

 brackish-water habits of the species. It should be borne in mind, how- 

 ever, that this faint rudimentary posterior lateral tooth does not exist in 

 all the typical species ; while it is obscurely represented in some marine 

 tertiary species of Corbula ; nor have I been able to find the posterior 

 pedal muscular scar in them all, more clearly defined than in some 

 marine tertiary species of Corbula. , 



Several of the other forms found in these Bear Eiver beds present 

 more marked peculiarities than are to be seen in Corbula pyriformis, 

 which may render it desirable to place them under separate sub-generic 

 groups ; for instance, that I have described under the name Corbicula 

 JDurkeei, which is a remarkable trigonal, thick shell, has the posterior 

 dorsal margins of the valves inflected, and the cardinal teeth directed 

 more obliquely backward than we see in the recent species, while its 

 elongated lateral teeth are nearly or quite smooth, instead of being 

 transversely striated. The posterior lateral tooth of its left valve is 

 also mainly formed by the beveling of the inflected margin, instead of 

 standing out as a distinct tooth. In some respects this shell is more 

 like Verlorita, from which, however, it differs materially in others. 



* In some interesting remarks, recently published by Mr. Henry Woodward of the 

 British Museum, in the Ann. and Mag., N. A., on the relations of Anisothyris to Corbula 

 and some allied groups, he mentions as one of the differences, that the latter have the 

 cardinal tooth in the left valve, while in Anisothyris it is in the right, and the socket rice 

 versa. This, however, is probably due to some typographical error, as there is certainly 

 no difference in the position of the cardinal tooth and socket, or in that of the cartilage 

 process with relation to each other,' or to the two valves, between Coroula and Anis- 

 othyris; both having the tooth in the right valve, and the socket and cartilage process 

 in the left. 



