Mr. K. Etliei'idge o/i Carboniferous Laintllibianchiata. 429 



oil the siiiuoiis margin of tlie anterior side, where, liowever, tliey 

 are only tlie edges of the hiniinre com[)Osing 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 scar 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 

 ^^l/ah'na, to be the scar of the insertion of the adductor of the 

 opposite Vcilve, similar to those Mytili 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 ^is the 

 double origin of the anterior adductor, c 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 

 Drei'ssena, 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 M. 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. Fo65. p. 4112. 



