284 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



If in imagination we chip away (an action always in progress) the whole 

 tipper part of the shell of TubicineUa, leaving only two or three zones of 

 growth at the base, we shall convert it into a Xenobalanus, with every internal 

 part and organ occupying the same relative position; for it should be borne 

 in mind that the shell of TubicincUa is lined close down to the basis by the 

 opercular membrane, and this is strictly comparable with the outer mem- 

 brane of the pseudo-peduncle of Xenobalanus. The body, as in TubicineUa, is 

 attached in a vertical position, with the longer axis of the thorax and of the 

 much elongated prosoma extending in the direction of the longitudinal axis 

 of the pseudo-peduncle. 



So far as I can see, the American specimens do not differ from 

 European. The color (in alcohol) is nearly black, and the ordinary 

 length is from 30 t'o 40 mm. The}' grow in close groups. The lo- 

 calities represented in the United States National Museum are North 

 Dennis and Woods Hole, Massachusetts, on the tips of the flippers 

 of Grlobiocephcdus intermedius, and Albatross station 2424, 36 41' 

 37" north latitude; 74 42' 15" west longitude, on porpoise. 



A form which may be called variety pattidus is represented by a 

 group of similar individuals, No. 2390, from the Fish Commission, 

 1875, labeled in Professor Verrill's hand " from tips of flippers of 

 GlobiocephaZus intermedium." The locality is doubtless New Eng- 

 land. They are light buff on one side, tinted with russet on the 

 other. Length 43 mm., diameter of shell 5 mm. (pi. G5, fig. 1). 



PLATYLEPADID SERIES. Oral opening not protected by a hood. Each 

 of the wall compartments has a median longitudinal sulcus or im- 

 prtssion. 



Genus PLATYLEPAS Gray. 



1S2.~>. ridti/lcpas GRAY, Annals of Philosophy, now por., vol. 10, p. 105, 

 monotype P. inih-lira Gray (!'. bi-ixc.i-lobata Blainville, according to 

 Darwin). 



1S32. CoUiincUlna BIVONA, Kffemeridi Sfi. e Lit. Sicilia, type C. bissexlobata, 

 pi. 3, fig. 1 (not seen). 



1S54. Platylepas Gray, DARWIN, Monograph, p. 424. 



Form conic, the base being decidedly larger than the orifice. Com- 

 partments six, each bilobed, at least inwardly, and produced so as 

 to form six midribs, which support the outwardly convex, mem- 

 branous basis. Sheath short. Basal and inner edges of parietes 

 sharply ribbed. Opercular valves occupying the whole length of 

 the orifice, their upper surfaces scaling off with growth. 



Type. P. hexastylos (Fabric-ins). 



Distribution. Tropical and warm temperate seas, partly embed- 

 ded in turtles, manatees, sea-snakes, and fishes. 



This genus consists, at present, of four species, of which only one 

 can be considered at all well known. The others are known by 

 single or very few lots. The essential feature of Platylepas is that 

 its membranous basis is supported by proplike midribs formed by 



