MUSEUM OF COMPARATIVE ZOOLOGY. .293 



ixmbonal angle of the hinge-margin, or be more or less adherent to the anterior 

 or posterior slope of this angle. They may have one anterior and one posterior 

 cardinal and lateral tooth in each valve, any one of which (or all in the genus? 

 Myonera) may be entirely absent. Beside the teeth the hinge is reinforced in 

 many cases by a buttress extending in a direction vertical to the valve from 

 the hidden surface of the hinge-margin, posterior to the umbonal angle. This 

 buttress may consist of the vertical plate above mentioned and a thickened rib 

 curving round in front of the posterior muscular scar, and then directed pos- 

 teriorly, becoming almost immediately obsolete. Or the posterior muscular 

 insertion may be elongate and narrow, and the buttress take the form of a 

 " clavicle " or myophore, elongated, parallel with the posterior hinge-margin 

 and separating the two posterior muscular scars. The muscles are not always 

 inserted upon the buttress, but may be above and in front of it. Its purpose 

 would seem to be that of strengthening the valve, almost always thin and 

 fragile, against sudden contractions of the muscles, and to support the cardinal 

 border, and especially the strong posterior lateral tooth found in many species. 

 When this tooth is found in a species which has no posterior lateral in the 

 other valve, the valve which has a tooth shows the buttress stronger than the 

 other, indicating its function as a support for the tooth; but when elongated 

 and clavicular there is little difference between the buttresses of opposite valves, 

 indicating that in such cases the function is the strengthening of the valve 

 itself. The presence of the buttress is, in my opinion, important only in a 

 minor degree, except when it takes the clavicular form; as in different species of 

 the same group, and even in individuals of the same epecies, its size and promi- 

 nence vary very greatly. Adriatic specimens of the typical species, C. cuspi- 

 data, show a strong buttress; British specimens of the same species often show 

 it faintly or not at all, while otherwise well developed. The names Necera, 

 Ehinomya, Aulacophora, Spathophora, and Tropidophora, among those which 

 have been applied to members of this group, by Gray, Adams, and Jeffreys, 

 are all preoccupied in zoological nomenclature, some of them several times 

 over. 



The characters of radiating and concentric sculpture in this group have no 

 more than a specific value; there are few species where they are not more or less 

 combined in the external ornamentation. The surface may be polished, smooth, 

 wrinkled, sulcate, or granulous. The anterior muscular scar is double or single, 

 the posterior scar double, in all the specimens I have seen where the scars could 

 be made out. 



The outer part of the scar in each case is due to the adductor of that end of 

 the animal, the other part to the insertion of the sphincter-like muscular band 

 described under Myonera paucistriata, further on. The observations made on 

 the anatomy of several species will be found at the same place. If the writer 

 has not been misled by contraction of the parts under the action of alcohol, the 

 group comprising Cuspidaria and Myonera would seem to be destitute of gills 

 or palpi, at least in the normal form of such organs. This, however, may not 



