45 



Order Pleurobranchiata. 

 Fam. Aplysiad^. 



Aplysia depilans 1 Geuoa. 



The small, polished, subglobular spirai (sinistral ?) nucleus or 

 apex of the older shell is, witli the subapical part of the sliell, co- 

 vered with a membranaceous reflection of tbe inner lip over its sur- 

 face, which is only slightly adherent to the surface of the shell aud 

 nucleus, and easily removed from it, but whįch gradually become 

 thicker ; the top of the shell appears to be absorbed, or more or less 

 obliterated iu the older specimens. 



According to Mr. Woodward, Mr. Hancock has observed in the 

 adult specimen two or three shells one within the other, likę the Loli- 

 gines or Sea slaves. 



Fam. Tylodinad^. 



Tylodina punctulata=7'. Rafinesąuįi, Philippi. 



Lingual membrane very broad, brown ; teeth small, uniform, very 

 numerous, in very numerous longitudinal lines, forming straight con- 

 tinued uniform lines across the membrane, with an indistinct centrai 

 line ; the tentacles subulate, slit on the outer side ; the lips are 

 produced and aoute on each side, and twisted, leaving a slight 

 cavity on the outer side of the tip ; the mantle is thin, free all 

 round the edge and slightly thickeued just within the margiu, 

 rather thicker aud more ffee over the frout of the back ; the gili 

 is single on the hinder part of the right side just under the mantle, 

 attached the whole of its length on the inner side by a centrai ridge 

 to the side of the body ; the outer side is furnished -vvith a rather 

 thick, rather zigzag centrai vessel, giving out pinnated vascular 

 brauches, nearly alternating with each other on each side of the great 

 vessel; the footis largerthan the mantle and shell, expanded, rounded 

 behind, truncated in frout aud slightly emarginate in the centre 

 under the mouth ; the sexual aperture not visible in the specimen 

 in spirits. Shell conic, patelloid, thin, slightly pearly -vvithin, with a 

 thin, hard, liorny periostraca^ which is produced beyond the edge of 

 the shell, and radiately coloured, in the dry statė brittle, hard, and 

 f ontracted ; the apex (of the shell) subcentral, mth a rather pro- 

 duced polished top, nucleus subglobose, -ffith a slightly convex spire 

 of one and a half or tvro rapidly enlargiug subconvolute whorls ; 

 aperture ovate, rather irregular, slightly dilated on the right side ; 

 cavity simple ; muscular scar subanuular, Avith an angular inflection 

 rather behind the middle of the right side, the form of the scar is varia- 

 ble, sometimes square, broad all round ; in the larger more developed 

 specimens the scar is rather horse-shoe shaped, being rather dilated 

 at the front part of each side, and the frout portion over the back of 

 the head is narrow, linear, aud transverse. 



The genus was first established by Rafinesąue in 1 8 1 4 ; Blainville, 

 who ouly knew it from Rafinesąue's imperfect descriptions, referred 

 it to the PateUoida, but Menke, Philippi aud Cantraine properly cou- 

 sidered it allied to Pleurobranckus, and especially Umbrella, and very 



