172 PALAEONTOLOGY. 



present the characters of Azara at all, but seems to agree almost exactly 

 with that of Corhula proper. From this fact, and its apparent fresh- 

 or brackish-water habits, I at first thought it might possibly find a place in 

 a newly proposed South American group for which Mr. Gabb used the 

 name Pachydon (but afterward named Anisothyris by Mr. Conrad, because 

 Pachydon had been previously used by Sowerby for another genus ), some 

 species of Avhicli closely resemble the shell under consideration; while the 

 South American type differs little from Corhula in its hinge-charactei's.* 

 Soon after, on informing my friend Mr. Conrad that I had found our shell 

 to differ in its hinge from Azara, and to agree almost exactly with Corhula, he 

 wrote to me that he had been studying specimens of the same, sent to 

 the Academy of Sciences from the original locality, and that he had pro- 

 posed, in manuscript, to make it the type of a new genus Anisorhynchus, 

 founded mainly on its brackish- or fresh-water habits, iV^e«ra-like form, and 

 supposed gaping posterior; and I adopted his name in a subgeneric sense, 

 in an extract from Dr. Hayden's Second Annual Report, tlien just ready to 

 go to press, as well as in the report itself, printed a month or two later. 



After examining hundi-eds of specimens of this shell, however, I have 

 failed to detect any evidence that its valves were in the least gaping; and, 

 as regards its Necera-Mke form, there is an objection to giving very much 

 weight to it as a distinguishing character, that is the fact that Corhula alce- 

 formis Gabb, from marine beds, has exactly the same general form; so that, 

 so far as yet known, the group Anisorhynchus seems to rest entirely ujjon 

 the apparent brackish-water habits of our shell. There are not known at 

 present any well-determined fresh- water living species of Corhula; but Dr. 

 Stoliczka says (Palaeont. Indica, III, p. 35) that there certainly are Indian 

 brackish-water species which cannot be distinguished from true Corhula, 

 excepting that they are thinner and Neara-Wke in form. 



From all that is therefore yet known in regard to the characters of this 



•Origiuallj-, AMJ«o//tJ/m, Courad, or Pachydon, G&hh, was supj»osed by both of 

 those authors to have an exterual ligament; but, on examining a series of the tvpe- 

 species, 1 found that it really lias an internal cartilage like Corhula, vfith only a more 

 oblique cartilage-process. Hence Mr. Conrad now rests the group mainly on its brack- 

 ish-water habits, the subspiral character of the beaks, and the presence, in some of the 

 specien, of an obscure rudimentary posterior lateral tooth. 



