PACHYLASMA GIGANTEIJM. 477 



this species, viewed outside, without particular care, would 

 be said to consist, like an Elminius or Chaniaesipho, of 

 only four compartments ; if the sheath alone were looked 

 at, there would be said to be six compartments ; but when 

 the walls, especially their basal edges, are carefully examined, 

 either internally or externally, the eight compartments can 

 be plainly distinguished. 



With respect to the affinities of this genus, we have seen 

 that both in the included animal's body, and, at an early age, 

 in the shell, it is extremely close to Octomeris : in the presence 

 of caudal appendages, and in the basis being calcareous, we 

 have a clear affinity with Catophragmus ; in the two lateral 

 compartments of P. aurantiacum, tending to become blended 

 together, we have some relationship exhibited to Chaniaesi- 

 pho, as is likewise shown in the structure of the second 

 and third pairs of cirri. If the genera of the Chthamalinae 

 were ranged in a circle, Pachylasma would be the point 

 of contact with the Balaninae. I must repeat, that it is 

 extremely unfortunate that when the shell alone of P. 

 giganteum is examined, it is hardly possible to separate this 

 genus from Balanus. 



1. Pachylasma giganteum. PI. 19, fig. ha — hd. 



Chthamalfs giganteus. Philippi (!) Enum. Mollusc. Sicilise, 1836. 



Shell and operculum dirty white : carino-lateral and la- 

 teral compartments furnished with similar alee. 



Hab. — Mediterranean ; Sicily ; deep water ; often attached to the Millepora 

 aspera, and sometimes associated with B.hdipiformis. Fossil in the tertiary 

 beds, near Messina. Mus. Brit., Cuming, Stutchbury, Lyell, Philippi. 



I owe to the kindness of Dr. Philippi, authentic speci- 

 mens of his Chthamalus giganteus, from the tertiary beds 

 of Messina : had this distinguished naturalist seen recent 

 specimens, or a fossil one with all the valves united, he 

 would no doubt have perceived that this species cannot be 

 classed with Chthamalus. I am also indebted to Sir Charles 

 Lyell for some magnificent specimens, which he himself 

 collected near Messina. 



