No. 531] GUNDLACHIA AND ANCYLUS 



179 



lumbia, was kind enough to undertake the removal and 

 mounting of radulse taken from specimens of the differ- 

 ent stages, as well as from the wild Ancylus. This was a 

 task of no little difficulty on account of the extremely- 

 minute size of the organ. The radulss of ancyloids, sep- 

 tates and Gundlachia were absolutely identical in ap- 

 pearance and in number of teeth, the formula 5- 10-1- 10- 5, 

 holding good for all. The uncinal teeth are not gradu- 

 ally modified from the laterals, but change abruptly and 

 form a distinct band on each side of the radula. The 

 lateral part of Stimpson's figure of the dentition of his 

 Gundlachia meekiana is imperfectly made out, and obvi- 

 ously inaccurate ; due doubtless to the fact that he had 

 only a few specimens and a not very powerful microscope. 

 Unfortunately his original material was destroyed in 

 the great fire at Chicago of 1871. 



An examination of the radula of a septate form, col- 

 lected in Nicaragua by Professor B. Shimek, showed a 

 similar radula but with one more uncinal tooth on each 

 side. In this case, unfortunately, while endeavoring to 

 transfer the minute object to a slide for permanent pres- 

 ervation, it mysteriously disappeared, and a trial with 

 a second specimen was no more successful. 



The form of the laterals is fairly well given by Dr. 

 Stimpson, and the rhachidian tooth is correct in his fig- 

 ure ; but the gradual modification and uncertain number 

 of the outer teeth of the radula do not agree with our 

 observations on the specimens from Ohio. Renewed 

 correspondence with Mr. Allen led to the preparation 

 of this paper, pending the continuation of his observa- 

 tions. 



Since the different stages of Gundlachia need to be 

 carefully discriminated, I have adopted the following 

 nomenclature for them. 



In the first stage, when the young shell has a laterally 

 compressed subcorneal shape without any trace of 

 septum, and is to all intents and purposes, concholog- 



