1887.] 



OX CYCLESTHERIA HISLOPI. 



13 



v 



animal is moving about. The connection between the body and 

 the shell is effected in a very limited space, immediately pos- 

 terior to the^umbones (see Pl. II, fig. 1). where the body sends 

 off opward a rather narrow neck-shaped prolongation. general ly 

 termed the cervical ligament, which is immediately continued 

 into the inner layer of the shell. From this prolongation, a 

 thickened rim exstends interiorly along the dorsal line of the 

 shell, both posteriorly and anteriorly. to the poiuts where the 

 free edges of the valves begin. Posteriorly, the rim terminates 

 rather abruptly in a somewhat dilated extremity, whereas an- 

 teriorly it gradually tapers to a very thin strip. Besides, the 

 body of the animal is connected to the shell by the strong ad- 

 dnctor mnscle traversing its anterior part, behind the mau- 

 dibles. 



In a living state, the body is more or less tinged with green 

 or brownish, and is always darker than the shell, save the an- 

 terior part of the head. which is very pellucid and light.-yellowish 

 like the shell. 



The body may be divided into four principal divisions, viz., 

 the cephalic part. or head, the cervical part. or neck. the pedig- 

 erous part. or trunk. aud the caudal part, or tail. In the 

 following pages. each of these divisions, together with their respec- 

 tive appendages, will be described separately. 



a. The Head. 



Pl. III, figs. 3—5.) 



As representing the head, or cephalic division, I regard 

 the anterior part of the body in front of the dorsal prolongation 

 connecting the body with the shell. This part consists of two well- 

 defined subdivisions, the one preoral. bearing the antennulæ, the 

 antennæ and the anterior lip. or labrum, and containing within it 

 the eye. the ocellus, the cerebral ganglion, and the anterior part 

 of the intestine, together with the æsophagus; the other pastoral, 

 having only a single pair of appendages, viz., the mandibles. 

 These two subdivisions of the head are above pretty well defined 



