GENUS — IBLA. 18 L 



yet known, and as these resemble each other in every 

 respect most closely, a generic description would be a 

 useless repetition of the full details given under Ibla 

 Cum'mgii. I have taken this latter species as the type, 

 from having, owing to the kindness of Mr. Cuming, 

 better and more numerous specimens. Ibla and Litho- 

 trya are the only two recent genera in which the body 

 of the animal is lodged within the peduncle ; but there 

 is no distinction of any importance, though useful for 

 classification, between the capitulum and peduncle ; and 

 these two parts, as we have seen, tend to blend together 

 in some species of Conchoderma and Alepas. The entire 

 absence of calcareous matter in the valves and spines of 

 the peduncle, at first appears very remarkable; but we have 

 seen a similar fact in Alepas, and there is an approach to 

 it in some varieties of Conchoderma aurita and C. virgata. 

 In all four valves of Ibla, the umbones, or centres of 

 growth, are at their upper points. The horny spines on 

 the peduncle, are the analogues of the calcareous scales in 

 Scalpellum and Pollicipes ; and in this latter genus, two 

 of the species have their scales, almost cylindrical, placed 

 irregularly, with new ones forming over all parts of the 

 surface, and not exclusively at the summit, — in which 

 several respects there is an agreement with Ibla. The 

 shape of the body (*. e. thorax and prosoma, PL IV, fig. 8 a) 

 is peculiar ; but it is only a slight exaggeration of what we 

 have seen in several genera, and shall meet again in some 

 species of Scalpellum. The presence of hairs on the outer 

 membrane of the prosoma is a peculiarity confined to this 

 genus amongst the Lepadidae, though observed in the 

 sessile genus, Chthamalus. The caudal appendages in the 

 I. quadrivalvis attain a greater length than in any other 

 species of the family, being four times the length of the 

 pedicels of the sixth cirrus. A far more important pecu- 

 liarity is the fact of the oesophagus, in both species, running 

 over or exteriorly to the adductor scutorum muscle, instead 

 of, as in every other species, close under this muscle. I 

 took great pains in ascertaining the truth of this singular 



