334 GENUS — LITHOTRYA. 



responding small knobs, on the upper part of the growing 

 surface of the carina. Moreover, considerable portions 

 of the inner surfaces of the scuta and terga, are rough- 

 ened with minute sharp, imbricated points, apparently for 

 the firmer attachment of the corium. The roughened 

 knobs at the rostral angles of the scuta, no doubt are 

 homologous with the teeth in a similar position on one or 

 both scuta in Lepas, and in some fossil species of Pollicipes, 

 as in P. validus. The other projections and roughened 

 surfaces are peculiar to Lithotrya. The growth of all the 

 valves is, as in Pollicipes, simply downwards. 



The Scuta are triangular, with their umbones or centres 

 of growth at the apex ; the tergal margin, as seen from 

 within, is either nearly straight or much hollowed out, 

 accordingly as the scuta simply overlap the terga, or are 

 received in a furrow. In some of the species there is a 

 distinct pit for the adductor muscle, and in others this 

 cannot be distinguished. 



Terga. — These present great differences in shape ; but 

 all appear to be modifications, (as seen internally,) of a 

 rhomboidal figure, which seems to be the normal form of 

 the terga in the Lepadidse. Of the lower part of the 

 valve, the whole exterior surface, with the exception of 

 a narrow ridge running from the apex down to the basal 

 angle, is hidden by the overlapping of the scuta, latera, 

 and carina. 



The Carina, in outline is triangular, with the basal 

 margin in some species extremely protuberant. In the 

 first four species, the internal surface is concave, in 

 L. truncata and L. Valentiana it is convex, with a central 

 raised ridge, and consequently the upper freely-projecting 

 portion of the valve, has a prominent central crest or ridge ; 

 in L. Nicobarica and L. RJiodiopus there is only a trace 

 of this ridge. The rostrum, as before stated, is always 

 very small ; it, as well as the latera, are most developed 

 in L. Nicobarica, and least in L. truncata and L. Valen- 

 tiana ; generally only a few zones of growth are preserved, 

 and from their being enlarged at their basal serrated rims, 



