GENUS PLATYLEPAS. 427 



Branchice : these consist on each side (at least in P. 

 decoratd) of a double fold, much less plicated than in Coro- 

 nula ; the fold nearest to the animal's body is rounder, and 

 not quite so large as the outer fold. 



Range and Habits. — The P. bissexlobata ranges from 

 the Mediterranean and west coast of Africa to the West 

 Indies, to the north-east coast of Australia, and, judging 

 from the name given by Chenu, to California : the second 

 species, P. decorata, inhabits the Pacific Ocean ; and the 

 third imperfectly known species, was deeply buried in the 

 skin of a sea-snake, off Borneo. The P. bissexlobata is 

 attached both to turtles and to the skin of the manatee : 

 some specimens in the British Museum, attached to the 

 latter, from the west coast of Africa, were entirely buried in 

 the skin, with the exception of the operculum. The speci- 

 mens of P. decorata, which I obtained at the Galapagos 

 Archipelago, were buried about half their depth in the softer 

 parts of the skin of the green turtle. I do not doubt that 

 the imbedment is effected in the same manner as in the 

 case of Chelonobia. 



Affinities. — This genus, in general structure and habits, 

 approaches closely to Coronula ; but it differs from all the 

 species of that genus in the following respects, which 

 appears to me fully to justify its generic separation : — The 

 outline is often oval, instead of circular ; the compartments 

 are not of equal sizes and shapes ; each compartment has 

 here only one inward furrow or fold, and this has its sides 

 pressed closely together, and is encased, but not obliterated, 

 by the sheath ; the rostrum is not quite medial ; and the 

 radii are narrow and nearly solid. The opercular valves, 

 again, are here more developed, and their microscopical 

 structure is different. The opercular membrane is less 

 constantly double ; and, what is much more important, the 

 muscles of the sack are not here spread out, and show no 

 tendency to become rudimentary and lose their transverse 

 striae. The cirri differ only in not being so broad and 

 flattened. The branchiae are not quite so much developed. 

 Finally, I may add, that in several of the above respects in 

 which Platylepas differs from Coronula, it approaches ordi- 

 nary sessile cirripecles. 



