320 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 3, 



Alcyonites PARASITICUM. 



Polypidom fleshy, parasitic, incrusting, mammillated. Cells nume- 

 rous, protuberant, scattered. Polyp. Tentacula short, cylindrical, 

 smooth, tapering to an obscure point. 



At the first view of the agate my attention was arrested by the 

 great size of the fibres contained within it, which vary from -^^ to -^ 

 of an inch in diameter, and on placing it beneath a microscopic power 

 of 100 linear, I was at once struck by their extraordinary hirsute 

 appearance. 



The mass of the polypidom appears to have been of a fleshy tex- 

 ture, and semi-transparent like that of Alcyonidiiim gelatinosum of 

 our own coast. It is built around the fibres of a species of Ferongia, 

 and the tubular fibres of the sponge are in many places in a beautiful 

 state of preservation. A portion of one of these is represented by 

 fig. 3. Plate VIII. Each fibre is not separately surrounded by the 

 fleshy substance and cells of the parasite, as would be the case if it 

 were a Gorgonia, but it often occurs that several fibres are included 

 within one circle of polyp cells, and the contained sponge fibres fre- 

 quently pursue a tortuous course, while the fleshy body of the Alcyo- 

 nite does not follow their contortions, but surrounds them in the 

 form of a large regular cylinder. 



The surface of the polypidom presents a strongly mammillated or 

 tuberculated appearance. The mammillae are not arranged in any 

 definite mode, but are scattered without order, thickly over the whole 

 of the surface. They vary in diameter from -^\^ to -^^ of an inch, 

 and are usually elevated about half the amount of their own diameter 

 above the outer surface of the polypidom. 



Within each of the mammillae, at a depth of about a fourth part of 

 their own diameter, there is a somewhat irregularly-formed globular 

 cavity, and usually within this there is a single small opaque black 

 mass, which from its comparative size and uniformity has probably 

 been the remains of the gizzard of the polyp*. 



The outer surface of the mammillae is usually more or less semi- 

 globular, but it is frequently the case that its apex is flat and some- 

 what depressed in the centre, so that a section of it in the direction 

 of its axis would present three sides of a right-angled figure, having 

 the angles slightly rounded off and the centre of the upper line 

 slightly depressed. Under these conditions it resembles closely the 

 appearances presented by the polyp cells of the recent Alcyonium 

 digitatum of our own coast, when the polyps are in a semi-extruded 

 condition, and the semi-globular appearance would be the condition 

 of the recent polyp when still further withdrawn within their poly- 

 pidom. These states of the mammillae of the fossil are represented by 

 fig. 4. Plate VIII. 



It is possible that this mammillated appearance of the surface of the 



* As the nature of this body cannot be determined with certainty, I have thought 

 it better to refer the animal to the estabhshed genus Alcyonites than to form a 

 new genus for its reception. 



