GASTEROPODA TECTIRRANCIIIATA. 



355 





Fif. 1C9.- 



The Bursatkli.f.s, Claiiiv., — 



The lateral crests are united in front, so as only to leave 

 an oval opening for the water to pass to the branchiic 

 ^vhieh are also destitute of a covering cloak. It is, how- 

 ever, probable that this genus should be allowed to lapse 

 into the Notarchus.* 



The Acehes, (Jkcra, MuUer) — 

 Have the brancbise covered like the preceding genera, but 

 their tentacula are so much shortened, widened, and sepa- 

 rated, that there seems to be none at all, or rather they 

 form together a large, fleshy, and nearly square buckler 

 under which the eyes are placed. Moreover, their her- 

 maphroditism, the position of their sexual organs, the 

 ^j) complexity and structure of the stomach, the purple liquid 

 which several of them shed, all approximate them to the 

 Aplysijc. The shell, in such as have one, is more or less 

 convolute, with a slight obliquity, without a visible spire, 

 and the mou:h has neither sinus nor canal; but as the 

 columella is convex and protuberant, the mouth has a 

 crescent-like shape, and the part opposite to the spire is always widest and rounded. When the shell 

 IS buried in the cloak, M. de Lamarck names the genus Bultea. The shell has few whorls, and is too 

 small to contain the animal. 



The JiuUcca aperia, Lam,, is an example which is found in almost every sea, where 

 it lives on oozy bottoms. Wlien the shell is [external], covered with a thin epidermis 

 and sufhciently roomy, M. de Lamarck allows them to retain the old name Bulla. 

 The Bulla liguarid, ampidln, Rud/iydafis are examples, [distinj^uished not only by the 

 characters of the shells, but by peculiarities in the armature of the stomach, which 

 couNists of two or three comparatively larji^e osseous pieces or jaws of dilferent shapes 

 in each. Of those of B. lignaria, Gioeni constituted a genus to which lie assi<^ued i^'^'- 1711— Buiiu?„npert.i. 



his own name ; it is the Tr'icia of Retzius, the Char of 

 Bruguiere, and dishf^ured our systems until the cheat 

 was detected by Draparnaud.J I restrict the term Accra 

 to such species as have no shell whatever, or merely a 

 vestiire of it behind, although the cloak has the external 

 form of one. The genus is the Dorirlium of Meckel 

 and Lobaria, Blainv. Tliere is a small species in the 

 Mediterranean {Bulla ciiriwsa, Cuv.), whose stomach 

 is as destitute of any armature as its cloak is of a shell, but the oesophagus is tlesliy and very thick. 



The Gasteroi^tlsron, Meckel, — 

 Appears to bo only an Acercs with the sides of the foot expanded into broad fins, by whose aid it is 

 enabled to swim, which it does in a reversed position. It also has no shell, and no stony apparatus 

 in the stomach. A very slight fold of the skin is the sole vestige of a branchial cover to be observed. 



The one species known {G. ilcchdii) is a i\tediterrauean Mollusk, about an inch long by two in breadth, when 

 its wings are spread out. 



Until a more ample anatomy has been made of it, we believe that it is in this order, and near to the 

 Pleurobranchus, that the singular genus 



Umbrella, Lam. {Ga^troplax, Blainv.) — 

 Shoidd be placed. The animal is a great circular Mollusk, whose foot exceeds by much the cloak, and 

 has its upper surface roughened with tubercles. The viscera are in a superior and central rounded 

 Iiart. The cloak is only visible by its slightly projecting sharp edge along the entire front, and on the 

 right side. Under this slight edging of the cloak are the branchia;, in lamellated pyramids, like those 

 of Pleurobraiichus ; and behind them is a tubular anus. Umler this same margin, in front, are two 



the back and tl.i 

 :[ilar network, m 

 brunchus.] 



of the lohc.s under the fort 

 iiliun of tijc Elysia ia nes 



