74 ' REPORT 1851. 



Zoology. 



Descriptions of two New Species of Nudibranchiate Mollusca, one of them 

 forming the type of a New Genus. By Joshua Alder and Albany Han- 

 cock. With the Anatomy of the Genus, by Albany Hancock. 



The Mollusks described were found at Falmouth by W. P. Cocks, Esq. The first 

 species noticed belongs to the rare genus Thecacera, of which only one species has 

 been before desrribed. The authors have named it Thecacera virescens, and cha- 

 racterize it as follows : — 



T. virescens. Body rather convex, smooth, of a light peach-blossom tint, blotched 

 with green anteriorly and posteriorly. Head with a plain subvelar margin in front. 

 Tentacles broadly laminated, retractile within sheaths with plain margins. Branchial 

 plumes five, green margined with white. A single row of obsolete tubercles encircles 

 the branchial region. Length i%ths of an inch. 



The next mollusk described belongs to the family Eolldidec, and constitutes a new 

 genus in that family, differing from EoUs in the position of the tentacles, and in 

 having a subdorsal anus, and a curious frilled membrane down the side of each bran- 

 chial papilla. It is thus characterized: — 



Oilhnna, n.g. Body elongated, limaciform. Head with four linear tentacles consti- 

 tuting two pairs, both subdorsal. Mouth with corneous jaws. Branchiae papillary, 

 clothing irregularly a subpallial expansion on the sides of the back ; a produced 

 membranous margin or fringe runs down the inner side of each papilla. Anus latero- 

 dorsal, situated towards the right side. Orifices of the generative organs separate. 



The species is named 0. nobilis. It is of a pale buff colour, with the branchiae of 

 a rich brown, their apices having a metallic lustre. 



The anatomy of the genus is given in detail by Mr. Albany Hancock. It cor- 

 responds with that of Eolis in its general features, but shows some very curious and 

 interesting modifications; the most remarkable of which is the great development of 

 the efferent or bran chio- cardiac vessels of the vascular system. These vessels, which 

 lie entirely within the skin, are conspicuous from the outside and show many rami- 

 fications passing into a large vessel which occupies the centre of the back behind the 

 heart. A small but very distinct vessel, in connexion with them, runs down the 

 frilled membranes of the papillae, showing the branchial character of these organs. 

 Another pecidiarity of this animal is found in the hepatic apparatus. The pyloric 

 extremity of the stomach receives two biliary ducts, one on each side of the intestine. 

 These, diverging as they leave the stomach, pass into the skin at the sides of the back, 

 where each opens into a wide hepatic channel that extends nearly the whole length 

 of the body, receiving branches from the glands of the papillae, &c. The anterior 

 portion of the great hepatic channels are apparently connected with two folliculated 

 glandular bodies, much and irregularly sacculated ; and amidst the cellular tissue at 

 the posterior part of the body there is likewise a glandular substance, folliculated and 

 apparently branched, in connexion with the hepatic canals within the skin. This 

 arrangement differs from that which prevails in the Eolididce, in nearly all the genera 

 of which the principal hepatic canals lie free in the visceral cavity, and there is a 

 medial posterior trunk. In this genus there is no such trunk, and the canals are 

 almost entirely within the skin. In this respect Oitho7ia resembles Her tiKsa, hut the 

 peculiarities of the digestive system alone distinguish it from that genus as well as 

 from every other member of the family. 



On the Branchial Currents ©/"Pholas and Mya. 

 By Joshua Alder and Albany Hancock. 



The existence of branchial cuiTents in the Bivalve Mollusca, received anddischarged 

 by difterent apertures, is now pretty generally admitted ; but exception has been 

 claimed for some genera and families v;hose anatomical structure is supposed to 

 present an insuperable obstacle to this arrangement ; their siphons, as it is thought, 

 having no communication internally. Amongst these are the Myadce and the 

 PholadidcE. 



Mr. Garner, in his excellent essay on the Anatomy of the Lamellihranchiata, takes 



