GENUS LITHOTRYA. 333 



sometimes hardly exceed in size, by not being moulted at 

 each period of exuviation. The latera overlie the carinal 

 half of the terga ; I presume that they are homologous 

 with the carinal latera in Scalpellum. Each successive 

 layer of shell forming the valves is thick, and extends 

 over nearly the whole inner surface ; hence the carina and 

 terga, and to a certain extent the scuta, either actually do 

 project freely much beyond the sack, or would have done 

 so, had not their upper ends been removed; for the upper 

 and old layers of shell, in most of the species, either scale 

 off or disintegrate and wear away. A rectangularly pro- 

 jecting rim, serrated by small teeth, is formed at the 

 bottom of each fresh layer of growth, along the external 

 surfaces of each valve (see upper part of fig. 1 V PL VIII.) 

 This structure, as well as that of the crenated scales on 

 the peduncle, is important, for by this means the animal, 

 as we shall presently see, forms and enlarges the cavity 

 in the rock or shell in which it is imbedded. 



The scutum overlaps either about one third or even one 

 half of the entire width of the tergum, and abuts against a 

 prominent longitudinal ridge on its exterior surface. In 

 L. truncata and L. Valentiana, this ridge on the tergum 

 being folded over towards the scutum, forms a conspicuous 

 furrow, receiving the tergal margin of the latter. In 

 L. Valentiana, there is a second furrow on the carinal 

 side of the tergum, receiving the upper end of the corium- 

 covered or growing surface of the carina. Besides these 

 provisions for holding together the valves, there are, appa- 

 rently, others for a similar purpose ; thus in each scutum, 

 under the rostral angle, there is a roughened knob-like 

 tooth, which touches the under side of the little rostrum, 

 and no doubt serves to give attachment to the mem- 

 brane uniting the three valves together. In some species, 

 the adjoining basal margins of the scuta and terga, where 

 touching each other, are inflected and roughened ; again 

 in L. Rhodiopits, the carinal angles of the terga are pro- 

 duced into points, and in L. truncata and L. Valentiana 

 into prominent roughened knobs, which touch two cor- 



