old females it becomes considerably decumbent, forming almost 

 a right angle with the carapace. Viewed dorsally or ventrally 

 (fig. 2), the carapace appears somewhat compressed, whereas 

 the head, by reason of the greatly developed fornices, is rather 

 broad compared to its length, clypeiform, and truncate anteriorly. 



The outer side of the carapace exhibits in adult specimens 

 a number of sharply marked concentric lines, indicating the suc- 

 cessive growth of the carapace. Whereas in young, though ovi- 

 gerous, specimens, only one or two such lines are present, their 

 number in old specimens may increase up to no less than 9 or 

 10, all with their centre in the anterior part of the dorsal side, 

 thereby finally assuming a peculiar gibbous aspect (see fig. 1). 

 These lines undoubtedly represent the free edges of as many ca- 

 rapaces of different size, lying one upon the other in an imbricate 

 manner (see fig. 2) ; and this remarkable structure, also occurring 

 in some other species of the genus, is caused by the imperfect 

 exuvation of the shell, whereof the inner coating only is cast 

 off, the outer chitinous lamella being retained upon the new de- 

 veloping shell. Also on the head similar lines of growth may 

 be traced, lying here however very close together and running 

 parallel to the" fornix (see fig. 2). 



The free edges of the valves are fringed throughout their 

 whole length with a row of ciliated setse of somewhat unequal 

 size (see fig. l). Anteriorly they are rather short and more 

 widely separated, but towards the middle of the inferior edges 

 they become rather elongate and densely crowded, forming there 

 a somewhat spreading fascicle. On the posterior edges they suc- 

 cessively become shorter and more spinelike (see fig. 3), having 

 each at some distance from the base a single rather strong se- 

 condary spinule, whereas their outer part is still setiform and 

 ciliated. On the edges of the older carapaces, or lines of growth, 

 these marginal setre are in the present species always found to 

 be worn away; this is however not the case with the European 

 species I. sordidus, where they are often found more or less 

 complete, at least on the outer lines. There is no other sculpture 

 to be observed on the carapace, saving the usual delicate punc^ 



