known as Gemma, Parastarte, and Psephidia. 475 



tcoth the sliell would be uniquely abnormal, for, as a rule, 

 when there is only one lateral tooth in each valve these are 

 the left anterior aud the right posterior. There is no excep- 

 tion in the Veneridae, and, if there are exceptions in other 

 families, I do not know of them. 



I regard these ridges and grooves as merely a modification 

 of the interlocking arrangement which is found on the dorsal 

 margins o£ many oilier genera, when lateral teeth are absent. 

 Dr. Dall has himself described such grooves in many members 

 of the Veneridae, and he knows very well that the grooves 

 always occur on the anterior margin of the left and the 

 posterior margin of the right valve, the opposite margins 

 being bevelled into loug ridges which tit into them. This is 

 exactly how they occur in Gemma, the only difference being 

 that the margins are more evenly bevelled into a median 

 ridge., so that they have rather a deceptive similarity to 

 lateral teeth; but this similarity should not have deceived 

 so experienced a conchologist as Dr. Dall. 



It appears, however, to have deceived H. C. Lea in 1842, 

 for Dr. Dall refers to his supposition that Venus gemma was 

 a Cyrena t and remarks, "This indicates that he had correctly 

 identified the teeth, which are like those of Cyrena, except 

 that the laterals in Gemma are relatively weaker, and the 

 cardinals are not bifid or grooved." This statement is very 

 misleading, for, though there are some species of Cyrena in 

 which the laterals are long narrow ridges, they are both in 

 the left valve, and they tit between the double laterals of 

 the right valve, so that the dentition is completely different 

 from that of Gemma. 



I have no hesitation in maintaining that Gemma has no 

 lateral teeth, for the following reasons: — (1) The ridges 

 which are supposed to represent them are in opposite valves ; 



(2) the anterior ridge is in the right valve, not in the lett ; 



(3) it is not received between two ridges in the opposite 

 valve, but merely into a long groove on the margin; 



(4) similar ridges and grooves may be seen on the margins of 

 Goniji/iiwi, Kately»a i and other genera. 



2. Parastarte, Conrad. 



The shell which is the monotype of this supposed genus 

 was described by Conrad in 1S45 as Astarte triquetra, but he 



subsequently separated it as a new genus under the name 

 of Parastarte* . His view uH its affinities was accepted by 



* True. Acad. Nat. Sci. Philadelphia, vol xiv. (1862). 



