428 BALANID/E. 



1. Platylepas bissexlobata. PL 17, fig. la — Id. 



Cohonula bissexlobata. Be Blainville. Diet, des Sciences Na- 



turelles, torn. 32, (1S21), Tab. 117, 

 fig. 1. 

 Platylepas pulchra. /. E. Gray (!) Annals of Philosophy, (new 



series), vol. 10, (1825). 

 Columellina bissexlobata. Bivona (fide Phllippi). Nuovi generi 



di Mollusch. (1S32), Tab. 3, fig.l.* 

 Cohonula Calieohniexsis. Chemi (!) Illust. Conch., Tab. 1, fig. 4. 



Shell with the transverse lines of growth conspicuous : pa~ 

 rietes permeated by pores ; sheath descending barely half- 

 way doion the parietes. 



Hah. — Mediterranean, attached to turtles. River Gambia, attached to 

 manatee. Honduras, attached to manatee. Moreton Bay, lat. 27° S., Australia, 

 apparently attached to the dugong of that coast. California (?) Mus. Brit., 

 Stutchbury, and Cumiug. 



General Appearance. — Shell generally much depressed, and broadly 

 oval or circular ; sometimes steeply conical. Orifice oval, generally 

 not large. Surface rather plainly marked by closely approximate 

 lines of growth, which with the lobed outline gives to the whole an 

 elegantly sculptured appearance : occasionally the longitudinal ridges 

 formed by the parietal septa are distinct. Basal diameter of largest 

 specimen three quarters of an inch. 



Structure of Shell. — I have nothing material to add to the generic 

 description. The midribs are not so prominent as in the following 

 species, and hence the basal membrane is less convex. The origin 

 of the midrib as a fold is very plain. The sheath descends barely 

 half-way down the walls, and is a little hollow on its under margin, on 

 each side of the midrib. Beneath the sheath the parietes are finely 

 ribbed (fig. 1 d), but to a variable degree. I may here remark, that in 

 the. specimens taken from manatee, on the coast of Africa and at 

 Honduras, the internal ribs extended further up and were plainer, and 

 the opercular valves seemed to be a little narrower than in the other 

 specimens, so that I at first suspected that they were specifically dis- 

 tinct, but I could make out no other than these small and variable 

 points of difference. 



Scuta : oblong (fig. 1 c), about twice as long a3 broad, with the 



* This memoir was published in the ' Effemeridi Scientifichc e Litt. per la 

 Sicilia,' according to the 'Bibliographia Zoologite et Geologic*;/ by Agassiz and 

 Strickland. 



