THE MORPHOLOrtY OV THE CIIEI EOSTOJF ATA. 289 



parietal muscles orig'inate tyjiieally from the hasal end of the 

 lateral walls. TIio ojiercnliim resembles that of F. pisci- 

 f or mis. 



I have observed the occurrence of typical parietal muscles 

 in five other species of F lustra. 



Membranipora. — The operculum and the parietal muscles 

 in M. membranacea, and other forms which have been 

 examined are essentially Fhistriiie in their arrangement. 

 They have been described by Nitsche (1871^ p. 42(3) and 

 others. 



Farciminaria hexagona, Busk.^ — The whole of the 

 very long frontal surface is membranous. The operculum 

 is like that of Fl u stra pise if or mis, with no basal sclerite, 

 and witli a marginal vertical flange prolonged into two 

 lateral points, into which the occlusor mnscles are inserted. 

 The orifice is markedly bilabiate, the operculum with the vesti- 

 bule opening precisely like a clasp-pnrse, of which the mai-- 

 ginal flange of the operculum forms the snpporting rim of 

 one half, while the distal edge of the zooccium supports the 

 other half. There are about six groups of parietal mnscles 

 on each side, the distal gronp probably acting as in Flu stra 

 pisciformis. 



The next few species are ordinarily placed with the 

 Cellularina, but their affinities appear to me to be with the 

 Flustrina. 



Diraetopia spicata, Busk '^ (fig. 5). — This is an instance 

 of a dendritic form, in which the lateral zooecial walls constitute 

 the greater part of the exposed surface of the funnel-shaped 

 zooccium. The frontal membrane is small, and is situated at 

 the broad end of the funnel, the edge of which is surrounded 

 by seven or eight hollow calcareous spines. These originate 

 from a massive calcareous ring which is notched externally at 

 the base of each spine to permit the cellular tissue of the 

 spine to communicate with that of the general body-cavity. 



• Busk (ISSl), p. 51. 



2 Busk (1S52), p. 35; MacGilliviay, ' Prodr. Zool. Vict.,' Dec. v, ISSO, 

 p. 33. 



VOL. 4G, PART 2. NEW SKRIKS. T 



