OCR Text |
Show 386 ~IOLLUSCA. observe three or fout· filaments which the animal can pt·otrude through these holes. The mouth is a short proboscis( 1 ). The PAnOLLN., Montf., have an almost circular shell, in which the holes are nearly obliterated, and there is a deep sulcus that follows the middle of the whorls, and is marked externally by a salient ridge; Padole bdquete, Montf., II, p. 114. STROMATIA, Lam. The shell more hollow, the spire more salient, and the holes wanting; otherwise resembling that of the Halyotides, which it thus connects with certain species of Turbo. The animal is much less ornamented than that of the Halyotides(2). In the following genera, which are separated from the Patellre, the shell is perfectly symmetrical, as well as the position of the heart and branchi:e.( 3). In the FISSURELLA, Lam., 'Ve perceive a broad fleshy disk undet· the belly, as in the Patell<E, a conical shell placed on the middle of the back, but not always completely covering it, and perforated at its summit by a small orifice, which affords at once an issue for the feces and a passage to the water, required for respiration; this orifice penetrates into the cavity of the branchi::e, situated on the fore part of the back, and in the bottom of which terminates the anus; a cavity otherwise widely opened above the head. A branchial comb is symmetrically arranged on each side; the eyes are on the external base of the conical tentacula, and the sides of the foot are furnished with a range of filaments( 4). (1) All the H.uYoTIDEs, Gm., except the imperforata and the perversa. This genus, although it has been denied, most certainly has its counterpart among the fossils. M. Marcel de Serres has described a species found in the cal· careous strata of Montpellier (Hal. Pl~ilberti), Ann. des Sc. Nat. tome XII, pl. xlv, f. A. (2) Halyotis imperforata, Gm., Chemn., X, clxvi, 1600, 1601. ( 3) They are the P ARACEPHALORA CEnvxco-nnANCHIJE BnANCIIIFERA, Blainv. ( 4) All the Patella: of the fifth division of Gmelin, except Pat. fissura; among others, Pat. grreca, List., 527, 1, 2;-P. nimbosa, List., 528, 4. We have a spe· cies in which the shell, at least six times the size of the mantle, simply surrounds the hole of the summit like a ring,-Fissurellu annulata, Cuv. GASTEROPODA SCUTIBRANCHIAT A. 387 EMARGINULA, Lam. The structure of the Emarginulce is similar to that of a Fissurella except that instead of the hole in the summit, the1·e is a small clef~ or eroargination in the anterior margin of their mantle and shell, which also penetrates to the branchial cavity; the margin of the mantle envelopes and covers a great part of that of the shell; the eyes are placed on a tubercle of the external base of the conical tenta:c~la, and the margin of the foot is furnished with a range of filaments( 1 ). PARMOPHORus, Lam. A great portion of the shell curved by the reflected margin of the mantle, as in the Emarginul<£; the shell itself oblong, slightly conical, and without hole or emargination; the branchi::e and other organs, as in the preceding gencra(2). ORDER IX. CYCLOBRANCHIATA(3). The branchire of the Cyclobranchiata resemble smalllamellre, or little pyramids forming a cordon more or less complete under the borders of the mantle, very nearly as in the lnferobranchiata, from which they are distinguished by the (1) Patella fissura, L., List., 543, 28, &c. The P ALl\URIA, Montf., must be allied to this genus. (2) Patella ambigua, Chemn., CXCH, 1918. N.B. Fissttrellw, Emarginulre, and Parmaphori are also found fossil. (3) M. de Blainville, who calls the order in which he places Doris CTCLODRANCRI! T.t., makes an order of the Patella:, and of the three preceding genera, which he names CEnvxconnANCHIATA, which he divides into the Retifera and the Branclti· fera. T~e Retifera are the Patellm, because he supposes that they respire through thcmedmm of a network in the cavity which is over their heau. I have vainly aought for it, however, nor could I discover there any other organ of respiration ~an the cordon of lamella: which extends round the under part of the margin of t c mantle. See A nat. of the Patella in my Mem. on the Mollusca. |