SUB-GENUS CREUSIA. 375 



6. Sub-Genus — Creusia. PI. 13, 14. 



Ckeusia. Leach. Journal de Physique, torn. 85, July, 1817. 



Compartments four, furnished ivith radii ; basis cup- 

 formed : attached to corals. 



Distribution, imbedded in corals throughout the tropical seas. 



Creusia is closely allied to Pyrgoma ; and had not this 

 genus already been adopted by several authors, I should not, 

 I think, myself have formed it ; though, no doubt, it har- 

 monises well with some of the other genera of the family, 

 which are perhaps all too intimately related. Creusia differs 

 from Pyrgoma only in the shell being separated by sutures, 

 into four compartments, with well developed radii : in other 

 respects, such as in general habit, in the cup-formed im- 

 bedded basis (not permeated by pores), in the opercular 

 valves, in the characters derived from the mouth and 

 cirri, there are no generic differences. This affinity is 

 more particularly evident when Creusia is compared with 

 the first few species of Pyrgoma : indeed, for a short time, 

 I was inclined to consider var. 10 of Creusia as identical 

 with Pyrgoma conjugation. With respect to the species of 

 Creusia, 1 confess I have been much perplexed in deter- 

 mining whether there be only one, or half a dozen. The 

 latter conclusion would almost certainly be arrived at if 

 only a few specimens were examined ; and it might naturally 

 be thought that some of the species were extremely well 

 marked ; the difficulty of drawing any line between varie- 

 ties and species, begins only when some hundreds of speci- 

 mens, from various parts of the world, are disarticulated, 

 cleaned, and carefully examined. Creusia, in this respect, 

 offers a striking contrast with Pyrgoma, in which nearly all 

 the species are strongly characterised. The shell differs very 

 little in the several varieties or species of Creusia j and the 

 most marked difference, namely, whether the walls are 

 permeated by irregular pores or not, seems certainly quite 



