12 



Gr. O. SARS. 



[No. 1. 



As to the structure of the valves, they are very thin and 

 fiexible, and rather pellacid. exhibiting under the microscope a 

 linely granular sculpturing, produced by nunierous small depres- 

 sed pits. The valves are composed of two distinct layers, the 

 outer one strongly chitinized, the inner forming a soft membrane, 

 lining the inside of the valves, botn connected by numerous small 

 transverse pillars, thus constituting within the substanee of the 

 shell a complicated system of anastomosing canals, through which 

 the blood circulates. The insertion of the pillars in the outer 

 chitinous coat produces a peculiar spotted appearance of the valves,. 

 more distinct in alchoholic specimens, the spots in such assuming 

 a more or less stellate form, especially on the posterior part of 

 the valves (see Pl. I, fig. 3, and Pl. II). 



Between the two lamellæ of the valves occurs on either side 

 a peculiar organ, to which an excretory fimction has generally 

 been ascribed. This is the so-called „shell-gland," which accord- 

 ing to its position, would thus, more properly seem to belong to 

 the shell than to the body itself. The organ is located in the 

 anterior part of the valves, partly encircling the insertion of the 

 adductor muscle, the greater part beiug extended however be- 

 low it and forming a well-defined oval figure, easy to distinguish 

 in fresh specimens (see Pl. I, fig. 3, and Pl. II). It is com- 

 posed of a somewhat flexuous canal, curled up into three irregular 

 convolutions, as shown in fig. 2, Pl. III, the walls of the canal 

 partly exhibiting a distinct cellular structure. 



B. The Body and its Appendages. 



The body of the animal is comparatively less massive than 

 in Estheria, and admits of being completely withdrawn into 

 the shell (see Pl. II, fig. 1). In living specimens, however r 

 the anterior part of the head, along with the antennæ, are gene- 

 rally found exserted from the anterior part of the shell, the 

 caudal plate being more or less projected beyond the inferior 

 or posterior edges; and this is invariably the case when the- 



