Order 4. 



GASTEROPODA TECTIBEANCHIATA. 



347 



Tig. ira. — BursMcUa Leacliii. 



The Bursatelles, Blainv., — 



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

 an oval opening for the water to pass to the branchice 

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

 ever, probable that this genus should be allowed to lapse 

 into the Notarchus.* 



The Aceres, {Akera, MuUer) — 

 Have the branchiae 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 

 complexity and structure of the stomach, the purple liquid 

 which several of them shed, all approximate them to tlie 

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

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

 and the mouth 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 Bullsea. The shell has few whorls, and is too 

 small to contain the animal. 



The Bul/cea aperta. Lam., is an example which is found in almost every sea, where 

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

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

 The Bulla lignaria, ampulla, and hydatis are examples, [distin^ished not only by the 

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

 consists of two or three comparatively larg-e osseous pieces or jaws of different shapes 

 in each. Of those of B. lignaria, Gioeni constituted a genus to which he assigned ^'«- iro.—Buiiia aperta. 



his own name ; it is the Tricla of Retzius, the Char of 



Bruguiere, and disfigured our systems until the cheat 



was detected by Draparnaud.] I restrict the term .'/cera 



^1'- 1; W to such species as have no shell whatever, or merely a 



vestige of it behind, although the cloak has the external 

 form of one. The genus is the Doridium of Meckel 

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

 Fig. 171 — Bulla liKnarii. Fig. \T2.-B. ampulla. Mediterranean (Bulla canwsa, Cuv.), whose stomach 



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



The Gasteropteron, Meckel, — 

 Appears to be only an Aceres ■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 obser\ed. 



The one species known (G. Meckelii) is a Mediterranean 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. {Gastroplax, Blainv.) — 

 Should 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 

 part. 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 branchise, in lamellated pyramids, like those 

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



* jiplysia viridis, Montsig., raised to a genus by Oben under the 

 name ol Actann, and ivhich is at least nearly allieil to the Eft/sin iimida 

 of Rissn. has been considered as a near ally of Aplysia, but from w ant 

 of a knouled{(e of the branchiae, I cannot classify it. [The branchlw 



cover the back and the superior surface of the lobes under the form of 

 avascular network, so that the true position uf the Klysia is next tu 

 Flacubranchus.] 



