MOLLUSCA. 213 



GENUS ALPIDIUM. Individuals in a single row round the 

 common centre. 



The thorax is cylindrical. The abdomen inferior, sessile, 

 and of the size of the thorax. Ovarium single, sessile, pla- 

 ced at the bottom of the abdomen, and prolonged perpendi- 

 cularly. Savigny divides the genus into two tribes. In the 

 first, the individuals are simply oblong, with an ovarium 

 shorter than the body, as A. ficus (Alcyonium ficus, Linn.) 

 In the second, the individuals are filiform, with an ovarium 

 longer than the body, as A. effusum of Savigny, tab. xvi. f. 3. 



GENUS DIDEMNUM. Individuals in distinct systems. 



The thorax is short and subglobular. The abdomen in- 

 ferior, pedunculated, and larger than the thorax. The anal 

 opening is obscure. The ovarium is single, sessile, and 

 placed on the side of the abdomen. D. candidum and vis- 

 cosum, from the Gulf of Suez, are the only known species. 



2. Branchial Orifice simple. 



The species form a thin fleshy crust on stones and sea- 

 weeds. The individuals are stellularly arranged in distinct 

 systems. The branchial orifice is circular and undivided. 

 The abdomen is sublateral, and fixed at the bottom of the 

 branchial cavity. The intestine is small, and the anus in- 

 distinct. The angles of the branchial meshes are without 

 papillae. 



GENUS BOTRYLLUS. Systems furnished with a central 

 cavity. The systems are prominent, and consist of one or 

 more regular concentric rows. The ovarium is double, 

 being attached to each side of the branchial sac. 



This genus is subdivided by Savigny into Botrylli stellati, 

 and Botrylli conglomerati. In the first, where the indivi- 

 duals are distributed in a single row, there are some species 



