FAMILIES AND GENEEA OE THE MADREPOEAEIA. 



15 



Bhizotrochus fragilis, Pourtales (Deep-Sea Corals, Illustr. Cat. 

 Mas. Comp. Zool. Harvard, 1871, No. iv. p. 17). Tlie septa of 

 the 1st and 2nd orders meet in tlie centre of the deep fossa, 

 and the corallum has an exquisitely fine pellicular epitheca 

 ornamented with curves and Vandykes, which is not to be distin- 

 guished from a wall. There is no theca in the ordinary sense. 

 The rootlets are small, and their cavities are continuous with 

 those of the interseptal loculi. 



RhizotrocTius tulipa of the same author (Hasslar Corals, 1874, 

 p. 39) has exsert septa besides the rootlets ; otherwise it re- 

 sembles the other species. 



Now the species are clearly divisible into those with a well- 

 developed rough epitheca and those without one, and in the latter 

 instance the wall is really epithecate. It is by no means sure 

 that the rough epitheca is not mural. 



Grenus Ehizoteochus, MM. Milne-Udivards ^ Jules Haime, 

 Hist. Nat. des Corall. vol. ii. p. 97 (1857), amended. 



Corallum simple, tall or short, cylindrical, compressed more or 

 less, aDd with a compressed or circular calice. Calice with a deep 

 fossa and thin septa, which are usually not exsert and never 

 much so. The columella is absent, and the septa either unite by 

 a few trabeculse or join across the axial space. The wall is very 

 thin, and resembles pellicular or opaque epitheca. Costse rudi- 

 mentary or absent. The epithecate wall is produced in the 

 form of rootlets, which are hollow and communicate w^ith the 

 visceral cavity. Corallum attached by the rootlets and base. 



Localities. — 'Recent. Mediterranean ; Florida seas ; Pacific. 



Genus Thtsanus, Duncan, Quart. Journ. Geol. Soc. vol. xix. 

 1863, p. 430, amended. 



Corallum simple, becoming free with age. Elongate, com- 

 pressed, low, pedicellate at one end of the long base. Calice 

 loDg, narrow, shallow, elongate, elliptical. Septa numerous, 

 radiating more or less from the end of the calice, which corre- 

 sponds with the basal pedicel, granular, minutely spinulose. 

 Columella small, parietal. Costse well developed, converging to 

 pedicel, granular, minutely spined. A groove may or may not 

 traverse the base. Epitheca variable. 



Distribution. — Fossil. Miocene : West Indies. 



