286 BULLETIN" 93, UNITED STATES NATIONAL MUSEUM. 



Distribution. Tropical and subtropical seas, on turtles (Caretta 

 caretta and Thalassochelys imbricatci) ; also on the African and 

 American manatees- the dugong, and a variety on Lepisosteus. 



The barnacle is much depressed and broadly oval or circular ; orifice 

 oval. Each compartment is divided by a median radial sulcus, which 

 does not reach to the orifice in small individuals. Surface elegantly 

 sculptured with close concentric ridges, and sometimes (especially 

 where worn) showing fine radial stria3. The outer layer is permeated 

 by fine pores. Radii rather narrow, transversely crenulated near and 

 at their edges. Basal and inner margins sharply septate. Sheath not 

 over half the length of the compartments, hollowed out beneath. 

 Each compartment bears an internal buttress or prop, the mem- 

 branous basis being stretched over them. 



Darwin says of it : 



Scuta oblong, about twice as long as broad, with the rostral end rounded, 

 rather narrower than the other end, and curled a little inward. Terga of 

 nearly the same shape and nearly as long as the scuta ; the carinal end is rather 

 more pointed than the scutal end of the valve, and when viewed internally, the 

 growing surface of this end is seen to be bluntly pointed. In both valves the 

 upper layers of shell usually scale off. 



The distribution of this species is wide, as would be expected from 

 the wandering habits of the sea turtles it lives upon. Moreover, it 

 appears to have a variety of hosts. All of the localities I have seen 

 reported are given in the reference paragraphs above. So far as 

 I know, it has not been reported from our west coast except by 

 Chenu, an authority of no great weight. 



There is a series of the t.ypical form, agreeing well with Fabri- 

 cius's figures, in the collection in the Academy of Natural Sciences 

 from Osprey, Florida, taken from Caretta caretta by Mr. Baker 

 (pi. 67, fig. 3). They measure up to 17 mm. in diameter. In profile 

 view the base is seen to be as convex as the upper surface in most 

 small examples, but less so in the larger. I took a series of smaller 

 specimens from a loggerhead turtle captured in Delaware Bay 

 (pi. 67, figs. 1-le). The form is very much depressed; diameter, 

 9 to 10 T ^ mm. The median sulci of the exterior are far less sharply 

 impressed than in the Florida examples, and the sharp septa within 

 the basal edge are longer. Both of these peculiarities, as well as 

 the considerable length of the internal props, may be due to imma- 

 turity, but from the worn condition and nearly uniform size of the 

 largest examples, I imagine that they are full grown, and represent 

 a slightly different race. 



Of the synonyms cited above, the names hexastylos, ~bisexlobata, 

 calif orniensiS) and concentrica were based upon the depressed form 

 figured in plate 67, figure 3, and are therefore, I believe, exactly 

 synonymous. P. pulchra Gray has not been described, but Darwin, 

 who had seen the type, placed it in the synonymy of U. liisexlobata. 



