Dr. J. E. Gray on some Families of Bivalve Shells. 401 



of the mantle, beginning with those most united and passing to 

 those most free, overlooking the fact, that the separation and 

 union of the mantle depend greatly on the habit of the animal, 

 and that when the habit of a genus approaches to that of a dif- 

 ferent family, the mantle agrees more in character with that 

 family than with its normal form; yet these authors would 

 scarcely have arranged the genera according to their habitation 

 alone. 



4. The position of the cartilage has been regarded as a cha- 

 racter of as much importance for the distinction of families as it 

 is for the separation of genei'a. 



In certain families, as Paphiadce, Crassatelladce, Corbuladce, 

 Pandorida, Anatinidce, Myada, Lasiadm, Leptonidae, Mactradce, it 

 is always internal and placed in a particular cavity separate from 

 the ligament. 



In other families, as Veneridce, Cardiadtie, Carditidts, Glossidce, 

 Astartidce, Solenida, Unionidce, &c., it is always external and mar- 

 ginal on the inner side of the ligament. 



But in TelliniddB and Lucinidce, which have the cartilage gene- 

 rally external and marginal and under the ligaments, some 

 genera have it placed in a triangular internal cavity distinct 

 from the ligament, as in Amphidesma and Loripes ; and have the 

 other characters of the animal and shell so like the typical genera 

 of the families, that it is impossible they can be separated from 

 them. And further, there are some genera in these families where 

 the cartilage is situated in such an intermediate manner, as to 

 be partly internal and partly marginal, so as to form a passage 

 between the above-named genera and those which are of the 

 normal form ; showing that in these families the situation of the 

 cartilage is of comparatively little importance except for the 

 distinction of the genera. 



A considerable variation in the structure of this part is to 

 be found in the family Arcada and its allies. In Area and 

 Pectunculus the cartilage is placed in angular lines on and partly 

 over the beaks. In one genus allied to Pectunculus it is placed 

 in a subinternal ti'iangular central cavity. In Nucula and its 

 allies it is placed in a regular internal central cochleate cavity, 

 as in Mactradce, which has caused those genera to be formed into 

 a family and placed near to it ; and in Solenella it is external, 

 marginal on the inside of the ligament, and furnished with a 

 rather large fulcrum like Solenidce, which has caused that genus 

 to be arranged near this family ; yet when the animals of these 

 families are examined and compared, they are found so nearly to 

 resemble each other, that they must be arranged together in one 

 group all characterized by the peculiar pectinated form of their 

 hinge- teeth. 



