GENUS PACHYLASMA. 475 



14. Pachylasma. Nov. Genus.* PL 19, 20. 



Chthamalus. Philippi. Enumeratio Mollusc. Siciliae. 



Compartments, token the shell is very young, eight ; when 

 mature, either six, or in appearance only four owing to the 

 close union of the lateral compartments : basis calcareous. 



Distribution^ Mediterranean, and New South Wales ; deep water. 



The two species here included form a very natural genus, 

 though, as far as the shell alone is concerned, at first sight 

 there is an unusual amount of difference between them. 

 This genus offers an instance of a case, far from uncommon 

 in nature, though so unfortunate for the systematist, in which 

 the most obvious and useful characters of a group are com- 

 pletely masked. When I first examined Pachylasma gigan- 

 teum,\ I did not doubt that it was a Balanus; and when I first 

 looked at P. aurantiacum, I thought, from there being in 

 appearance only four compartments, that it was an Elmi- 

 nius ; in neither case, from the absence of alae to the 

 rostrum, did I even suspect that the species belonged to the 

 sub-family of the Chthamalinse. But when I examined the 

 included animal's body, I found, in both species, the labrum 

 bullate, not notched, with the palpi small, and the mandibles 

 with their lower teeth not laterally double. Again I found 

 in the third pair of cirri only the basal segments thickly 

 clothed with spines ; and lastly, there were caudal appen- 

 dages. Now these characters are pre-eminently those of 

 the Chthamalinse ; in fact, they are those met with in the 

 typical genus Octomeris, with the exception of the presence 

 of caudal appendages, and these occur Catophragmus, — a 

 genus standing next to Octomeris, and in no other genus 

 of sessile cirripedes. Moreover, if we look to the shell of 

 Pachylasma, the absence of pores in the parietes, or at least 



* Tla%vQ thick, and eXac^a, a valve. 



f Dr. Philippi called this species a Chthamalus ; giving this generic name 

 from an examination only of the separated valves in a fossil condition. 



