74 A MONOGRAPH OF THE 



of them been broken off. This supposition is supported 

 by the fact, that in the large species of the closely allied 

 species, Euplectetta aspergillum, Owen {Alcyoncellum, Quoy 

 et Gaimard), no such large opening exists ; the distal ter- 

 mination of the sponge being permanently closed by a 

 strong network of spicula, in the areas of which the oscula 

 are placed, the whole of them being congregated at the 

 apex of the sponge. 



Nor can I agree with Montagu, that the structure 

 of the skeleton is similar to that of Tethea lyncurium, 

 which consists of numerous slightly curved fasciculi of 

 spicula, radiating from the base to all parts of the external 

 surface of the sponge, while in P. mammillaris the skeleton 

 of the basal mass consists of a plexus of contorted anasto- 

 mosing fasciculi, with short bundles of skeleton spicula 

 disposed on the inner surface of the dermal crust of the 

 sponge, at about right angles to its plane, and which ter- 

 minate acutely. In the adult sponges these fasciculi are 

 continued through the dermal surface, and from the series 

 of primary defensive spicula, and are exceedingly numerous 

 in the Larne Lough specimens, while in the immature 

 sponges they are very sparingly produced. 



I received from Mr. Barlee, in June, 1856, two separate 

 fistulas of this species, neither of them had the basal termi- 

 nation. They were rather longer and larger than the 

 fistulae of the specimens from Larne Lough, and their distal 

 terminations were more attenuate and rounded, but struc- 

 turally they exhibited no difference whatever from the 

 Irish and Guernsey specimens of the species, and I subse- 

 quently received from the same gentleman in June, 1858, 

 four small specimens of this species, which he dredged up 

 at Shetland ; they were all more or less depressedly conical, 

 and they varied in diameter at the base, from four lines to 

 an inch. The smallest, four lines in diameter, had not a 

 single mammilla on its surface, but the other three were 

 amply supplied with them. They were all short, and had 

 more of the characters of mammas than of fistulae, and the 

 same peculiarities were observable in a specimen of about 

 an inch in length by half an inch in width, that my friend, 



