34 BLOOMER : ANATOMY OF CERATISOLEN AND SOLECURTUS. 



In an extended condition the proximal portion of the siphon is of 

 considerable length, and nearly of a uniform size, but in a contracted state 

 it is larger at the centre and compressed at the ends. The free portions, 

 when fully extended, are likewise of considerable length. One specimen 

 measured G cm. along the median line from the posterior adductor muscle 

 to the anterior adductor muscle, and the proximal portion of the siphon, 

 though not fully extended, was of the same length. Part of the free por- 

 tions were missing, this being usually the case, as presumably being due 

 to the action of the preservative medium on the arrangement of the 

 circular muscles forming the lateral ribbings, and making them of a some- 

 what brittle nature. In another instance the proximal portion of the 

 siphon was in a very contracted condition, and only measured 2 cm. ; 

 while the free portion of the exhalent chamber measured 5.5 cm., and the 

 inhalent one G.l cm. 



There is no fourth aperture. 



The foot is very large, being deep and muscular. The periostracum 

 passes from the mantle lobes to the edges of the valves. 



The dorsal integument posterior to the teeth of the shell is thin, and 

 it is to be observed that the teeth deejDly penetrate into the viscera. 



The gills are long, and extend in a more or less folded condition to the 

 posterior end of the proximal portion of the inhalent siphonal chamber, to 

 which they are attached. 



The inner parts of the bases of the gills are joined together, forming 

 the division between the two siphonal chambers. 



Musculature. 



i. The Pallial Muscles. — ^The muscles along the edges of the mantle lobes 

 form anteriorly deep bands, which diminish in width as they pass pos- 

 teriorly. From the ventral border of the anterior and posterior margins 

 of each lobe proceed two muscular bands, which cross the piece of muscular 

 tissue connecting the ventral edges of the mantle lobes, and converging 

 towards each other, meet and vmite at the centre of the tissue. They 

 evidently act as retractor muscles for the purpose of drawing together 

 the ventral parts of the mantle lobes. The walls of the proximal portion 

 of the siphonal chambers (Fig. 4, Ex. tS\ & In.S\) are thick and muscular, 

 and continuous with the dorsal integument and the mantle lobes. Chiefly 

 laterally they give rise to two large muscles which pass through the mantle 

 lobes, and from a large surface adhere to the valves of the shell. These 

 undoubtedly act as siphonal retractor muscles. 



On the inside of each lateral part of the proximal portion of the siphon 

 and between the two chambers, is a large muscular ridge (Fig. 4, S. R.), to 

 which adhere the outer sides of the bases of the gills. At the posterior end 

 of the proximal portion of the siphon it decreases in size, and continues 



