Mr. W. Clark on the Genus Assiminia. 57 



species. A, anterior end ; B, jjosterior end ; a, posterior ridge ; 



b, anterior excavation of cardinal tooth of right valve, b; a + , pos- 

 terior slope ; /j + , anterior ridge of cardinal tooth of left valve, a; 



c, unibonal ligament. 



Fig. 4. Diagram exhibiting vertical section of the cardinal teeth of Unio 

 littoralis. a, single tooth of right valve, b, clasped by the two 

 teeth, b, of left valve, a. The remaining letters have the same 

 references as in tig. '6. 



Fiff. 5. Aiit/iracosia acuta, Sowerby. Cast, exhibiting adductor muscular 

 impressions, a, a — scars of the anterior set of foot-retractors, b — 

 and pallial line, c. 



Fig. 6. Anthracosia Smithiil Brown. Testiferous specimen, exhibiting 

 the cartilage, a. 



Fig. 7- I'liio distortus, liean. Jurassic. Cast in Mr. Bean's Museum, ex- 

 hibiting anterior adductor muscular imjjression, a — scars of tho 

 anterior set of foot-retractors, 6 — supplementary scar, c — and a 

 portion of pallial line, d. 



VII. — On the Genus Assiminia. 



[We have received a further communication on this subject from 

 Mr. Clark, in reply to the observations of Dr. Gray in our last 

 Number, which we must decline publishing, the question havino- 

 been sufficiently discussed in our pages to enable the reader to form 

 his own opinion on the matter in dispute. We cannot however, 

 in fairness to our correspondent, refuse insertion to the two fol- 

 lowing paragraphs, which we trust will terminate the correspondence 

 on this subject. — Eds.] 



"With respect to the generic maxims propounded by Dr. Gray," 

 Mr. Clark observes — " I dissent from them, — and disagree w'ith 

 Dr. Gray's restriction of the number of the species of a genus, except 

 under more stringent conditions than he has stated. Dr. Gray's 

 comparison of Assiminia and Truncatella is decidedly incorrect, and 

 I maintain the position that the so-called Assiminia Graijana is 

 malacologically a well-marked Truncatella. 



" I would also draw attention to the unfoir manner in which 

 Dr. Gray has commented on some of my statements : there is a 

 studied reticence in his observations on important points, the corre- 

 lative incidents of which are passed over without that notice which 

 would have supplied corrections and explanations. To give an in- 

 stance : in these ' Annals ' for October I observed that I was ' not sur- 

 prised that my logic should not find favour with one who consideis 

 that a genus must be restricted in the number of its species, however 

 similar these may be in every essential character.' Dr. Gray quietly 

 suppresses every allusion to the words which I have marked in 

 italics, probably from a fixed idea that essential characters are of no 

 importance in the establishment of genera. If I am correct in this 

 supposition, I must still decline to entertain the belief that such 

 views are held by modern zoologists, notwithstanding Dr. Gray's 

 repeated assertion that such is the case." 



