00 DR J. H. ASHWORTH 



• 



of the conditions in the areas where the larvse recorded l>y Muller and Blouhmann 

 were taken that the adults of P. atlanticus are not confined to deep water, and the 

 occurrence of the larvse in shallow water near Cape Comorin, while not conclusive, 

 lends support to this view. 



Description of the Larvse (Plate V). 



The six "larvse agree closely in size and structure, and are evidently all about 

 the same stage of development. 



The shell-valves (fig. 11), which are unequal, are free from each other all round 

 their margins, there being no hinge ; they are connected only by the muscles — 

 especially the occlusors, and the body-wall. The dorsal valve, which is almost like 

 a watch-glass in form, is sub-circular and varies in the different specimens from 

 •39-"43 mm. in length, and -43- - 47 mm. in breadth. The almost flat ventral valve, 

 which has a peduncular sinus in the posterior middle line, is •32- , 34 mm. long, 

 and - 39-'42 mm. wide. The maximum dorso-ventral diameter of each of the three 

 larvse in which it was measured is '075 mm. These larvse are about the same size 

 as those described by Professor Blochmann and that by Dr Yatsu, and slightly 

 larger than Muller's original examples. Both valves are chitinoid and transparent, 

 and exhibit no trace of calcareous substance. The edge of the dorsal valve, and to 

 a less extent that of the ventral valve, are yellowish brown in colour. The shell- 

 valves vary in thickness in different portions from about 3 to 8/x. The chitinoid 

 shell, which cuts easily in paraffin, is covered on its external surface by a definite, 

 though very thin periostracum, which is continued over the edges on to the inner 

 face of each valve, where, overlying the thin margin of the mantle, it extends 

 inwards until it reaches the thickened zone of the mantle by which it is evidently 

 formed. In surface view the periostracum presents a shagreened appearance. 



The mantle is for the most part colourless, but in its postero-lateral margins 

 exhibits some yellowish-brown pigment, and along the margin of the peduncular 

 notch bears darker brown pigment-granules. The mantle is very thin over the 

 greater part of its extent, but presents a thickened zone in its lateral and anterior 

 portions in both valves, but especially in the dorsal valve (fig. 17). The posterior 

 mantle-margin is thin in the dorsal valve, and in the ventral one forms a narrow 

 strip posterior to the peduncle. In the thin portions of the dorsal and ventral 

 mantle-folds, the outer surface of which is, of course, applied to the shell, there 

 are numerous gland-cells containing large granules ; these cells no doubt secrete the 

 shell. In the thickened zone of the mantle there are, besides gland-cells, more 

 numerous epithelial cells and some muscle-fibres ; the chsetse are implanted in this 

 region, which, as already stated, secretes the periostracum. 



There are five pairs of principal chsetse, as described by Muller (1860). Addi- 

 tional details regarding them may be given here. The two anterior chsetse of each 

 side are slender and flexible, and are about "15— "16 mm. long. Each has a simple 



