340 



GILMAN A. DREW. 



glossy, and quite transparent. They do not correspond to the 

 adult valves in shape (fig. 50), and they do not have the long, 

 straight hinge-line of the prodissoconch of Yoldia (Text- fig. K) . 

 The hinge-line is not very definitely marked off from the rest 

 of the shell, but it can be distinguished as a nearly straight 

 or slightly curved portion on the dorsal margin (fig. 36). The 

 difference in the shape of the prodissoconches of Nucula and 

 Yoldia is quite marked, more marked than might have been 

 expected for forms so closely related, when there is so much 

 resemblance between the prodissoconches of many Lamelli- 

 branchs (6). They both conform to the same type, how- 

 ever. 



At first the valves are thin and have neither cartilage pit 

 nor teeth. Soon after casting, a little knob of cartilage (fig. 

 36, ca.) makes its appearance near the middle of the hinge- 

 line. The teeth do not form until a much later stage (fig. 46). 

 About the time that the fifth pair of gill plates are formed, a 

 tooth appears on each valve in front of the cartilage pit. 

 This is soon followed by another, which is added anteriorly. 

 The teeth posterior to the cartilage pit begin to appear about 

 the time that the third tooth anterior to the cartilage pit is 

 formed. New teeth in the posterior series are added pos- 

 teriorly. Only about half as many teeth are formed posterior 

 to the cartilage pit as anterior to it. Apparently as long as 

 the shell continues to grow in size new teeth are added. 

 Shells of fully grown specimens are about 4 mm. long, but 

 they sometimes occur nearly 5 mm. long. 



Each shell-valve is very convex (figs. 50 and 51), slightly 

 oblong, and moderately thick. The beaks are directed pos- 

 teriorly and placed far back on the shell. This gives an 

 appearance quite the reverse of most Lamellibranch shells, 

 which have the beaks nearer the anterior than the posterior 

 ends, and directed forward. The cuticle of the shell differs 

 in different specimens from horn colour to dark brown or 

 nearly black. It may be considerably broken near the beaks, 

 but it is generally quite perfect and smooth. Unlike most 

 Lamellibranchs, the shells of this species contain so much 



