Mr. R. Etheridge on Carboniferous Lamellibranchiata, 429 
on the smuous margin of the anterior side, where, however, they 
are only the edges of the laminz composing the shell—a fact 
extremely well shown in one of the typical specimens (No. 219) 
in the ‘ Fleming Collection,” Edinburgh Museum of Science 
and Art, for access to which I am indebted to the kindness of 
my friend Dr. Traquair. In the right valve of some speci- 
mens the hinge-plate appears to be thicker and more highly 
developed than in the left. In old specimens the pallial line 
becomes exceedingly well marked, a series of pits indicating 
the points of attachment of the mantle-fibres. In the larger 
number of specimens of J. crassa, the posterior sides of the 
shell are usually broken away, indicating that they were thinner 
and more delicate than the anterior. At certain localities the 
thickness of the valves of this species is remarkable, so much 
so as to leave, in some instances, little room for the mollusk. 
The sear of the anterior adductor is placed well within the 
umbonal cavity, and in both valves is either single and pit- 
like, or may consist of two deep pits separated by a ridge, and 
often bounded posteriorly by a raised rim or margin. Over 
each, and a little posterior to the single or double impressions, 
is a third and smaller pit, which was ascertained by Prof. 
M‘Coy*, as stated in his emended description of the genus 
Myalina, to be the scar of the insertion of the adductor of the 
opposite valve, similar to those Myti/i with rostral plates, and 
not that of one of the pedal muscles, the larger impression in 
each valve being therefore only the origin of the respective 
adductors. Immediately in front, and within the angle formed 
by the hinge-plate and the anterior margin, is another, shal- 
lower depression, from which a depressed and more or less 
interrupted line runs in many specimens across the cartilage- 
area, sometimes even interrupting the furrows themselves. 
These points are well shown in figs. 1 & 2 (representing the 
interior of portions of a right and a left valve), where bis the 
double origin of the anterior adductor, ¢ the insertion of the 
adductor of the opposite valve, d the shallower impression from 
which runs the groove e across the cartilage-furrows. The 
late Prof. Pictet noticed the resemblance of the rostral septum 
characteristic of the genus Myalina to the shelf of the living 
Dreissena, on which is supported the anterior adductor : 
it may be that the elevated rim or margin, which I have 
described above as bounding the anterior scars of J. crassa, 
may still further tend to unite the two genera. From the 
condition of the specimens, I have been unable to study the 
* Brit. Pal. Foss. p. 492. 
