MUSEUM OF COMPARATIVE ZOOLOGY. 293 
umbonal 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 wpon 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 species, its size and promi- 
nence vary very greatly. Adriatic specimens of the typical species, C. cusp7- 
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 Nevra, 
Rhinomya, Aulacophora, Spathophora, and Tropidophora, among those which 
have been applied to members of this group, by Gray, Adams, and Jeffreys, 
are all preoccupied in zodlogical 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 
