Nudibranchiate Mollusca. 291 



normal form of Thecacera, with which it agrees in the sheathed 

 tentacles and the plain frontal veil without filaments or tubercles. 

 It seems to bear much the same relationship to T. pennigera as 

 Polycera ocellata and P. Lessonii do to P. quadrilineata: 



Two specimens were found by Mr. Cocks in March 1849, at 

 low-water mark on the oyster bed at Bar Point, Falmouth. 



The next novelty we have to describe is still more interesting. 

 It belongs to the family Eolididce, but presents peculiarities that 

 forbid its being associated generieally with any known form of 

 that family. It will be necessary therefore to establish for it a 

 new genus. 



OlTHONA*, n. g. 



Body elongated, limaciform ; head with four linear tentacles, 

 constituting two pairs, both subdorsal ; the anterior pair, corre- 

 sponding to the oral tentacles of Eolis, being situated consider- 

 ably behind the lips. Mouth with corneous jaws. Branchiae 

 papillary, clothing irregularly a subpallial expansion on the sides 

 of the back and meeting posteriorly ; 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 ; situated below the tentacles on the 

 right side. 



This genus differs from Eolis in the anterior pair of tentacles 

 not being placed on the lip, in the subdorsal position of the anus, 

 and more especially in the curious frilled membrane that runs 

 down the side of each branchial papilla. The papillae are also 

 much more firmly attached to the back than in Eolis, and the 

 apertures of the sexual organs are disunited. The anatomy also 

 shows several interesting points of divergence. 



O. nobilis. Body pale buff or whitish, smooth ; tentacles long, 

 broad at the base, and tapering to a fine point at the apex ; 

 not wrinkled or laminated j both pairs nearly equal in length. 

 Branchiae very numerous and crowded, commencing behind 

 the tentacles and set without apparent order on the sides of the 

 back on a subpallial expansion which is considerably produced 

 posteriorly. They are linear-conical and rather compressed, 

 particularly towards the base; the lateral fringe wide and 

 strongly waved : the central vessel is of a rich dark brown, the 

 sheath and waved membrane of a transparent buff-colour : 

 the apices have an iridescent or metallic lustre, which is ob- 

 servable also on the back. The foot is long and lanceolate, 

 rounded in front and produced into a fine point behind ; the 

 margins thin. Length 2 inches. 



* Oithona (the virgin of the wave), one of the heroines of Ossian. 



19* 



