STATR GEOLOGIST. 27 



portion or ormosteg if e, and a simple anterior cephalostegite\ but it 

 seems much more proper to consider that portion of the shell 

 which covers the pericardial space, and is the point of attachment 

 of the powerful muscles of the abdomen and of the membranous 

 walls of the pericardium, as a distinct portion of the carapace, as it 

 often evidently appears through the presence of a distinct suture, 

 or, in its absence, through the peculiar sculpture of the shell. In 

 such case it might also be proper to distinguish two regions on the 

 lateral appendages of this dorsal shield, an upper and a lower, 

 separated by the more or less obvious line, extending from the 

 union of the lateral lines of the dorsal and cephalic shield in 

 nearly a straight line toward the posterior portion of the shell, 

 and indicating the insertion of the muscles which move the feet 

 and post-abdomen. The lateral walls of the pericardial space 

 are the shell-walls themselves, and the floor is formed by a mem- 

 brane supported on, and investing in part, the strong muscles 

 which connect the abdomen with the upper anterior part of the 

 dorsal plate. Thus a space is left between the pericardium and the 

 intestine which is occupied by a special blood sinus leading toward 

 the posterior and lower part of the abdomen. The posterior wall of 

 the space is formed by a chitinous partition which bounds the 

 brood space, or its homologue, and is connected by chitinous 

 processes (stutzbalken) with the outer skeleton. The anterior, on 

 the other hand, is only bounded by the supporting ligaments of 

 the abdomen above described and membranous partitions. As 

 usually described, the heart lies suspended in the cavity thus de- 

 fined, by slender muscular threads, more or less like those of the 

 heart of Corethra larvae and the like; and such seems to be the 

 case at first, but a more careful study shows that this is far from 

 correct. On the contrary, the chief supports of the heart are 

 membranes which, seen in cross-section with the attached grains 

 or blood globules, assume the appearance of exceedingly slender 

 structureless threads. The action of re-agents indicates that these 

 supposed threads are not muscles, but composed of connective 

 tissue; while by changing the focus the sharpness of the line is 

 frequently not altered, but its relative position is changed^ — a simple 

 test which often serves to dispel an illusion of this sort. That 

 there are some threads of the character above mentioned is not to 

 be doubted, as in connection with the valves of the heart; but the 

 proper support of the heart is found in the membranes which 

 invest it in part, and are reflected upon the walls of the shell and, 

 anteriorly, of the intestine. It is not yet possible to fully describe 



