350 GENUS LITHOTRYA. 



Penis, elongated; vesiculse seminales extending into the 

 prosoma. I noticed the ovigerous frsena only in L. trun- 

 cata; here they were large, with an almost bilobed out- 

 line ; the margin and whole lateral surface being covered 

 with elongated cylinders, finely pointed, but not enlarged 

 at their extremities, as are the glands observed in most 

 of the other genera. 



Colours. — The posterior thoracic segments, the pedicels, 

 the anterior and dorsal surfaces of the segments of the 

 cirri, the caudal appendages, and the outer sides of the 

 trophi are, in most of the species, more or less mottled 

 with dark purple ; parts of the interior surfaces of the 

 valves in some of the species are coloured fine purple. 



Geographical Distribution. — The species are found all 

 round the world in the tropical seas ; this fact may have 

 some connection with the presence of soft coral-reef lime- 

 stone and of massive corals in these seas. The presence, 

 however, of L. cauta on the shores of New South Wales, 

 shows that the genus is not strictly tropical. 



Affinities. — Lithotrya is a well-pronounced distinct 

 genus ; although there is a considerable difference in the 

 shape of the valves between L. dorsalis and L. Valentiana, 

 at the opposite extremes of the genus, the strict uniformity 

 of the internal characters shows that there are no grounds 

 whatever for any generic separation ; moreover, L. fflio- 

 diopus neatly blends together these extreme forms. Indeed 

 it is not easy to imagine a better marked series of transi- 

 tional forms, than those presented by the terga, in passing 

 from L. dorsalis through L. Nicobarica, L. Rhodiopus, 

 and L. truncata, to L. Valentiana. Lithotrya has most 

 affinity to Scalpettum villosum or to Pollicipes spinosus and 

 _P. sertus; though the affinity is far from close. In these 

 two species of Pollicipes, we have seen that large irregular 

 calcified spines are formed at the base of the peduncle, 

 whereas in the other Pedunculata the scales or spines 

 are formed exclusively round the upper margin of the 

 peduncle. Lithotrya, as has been remarked by Sowerby 

 and other authors, exhibits some affinity to the sessile 



