Gasreroropa.] UPPER PALZOZOIC MOLLUSCA. 521 
Gth Class. GASTEROPODA. See page 288. 
5th Order. TrcripraNncHiATaA. See page 289. 
Genus. POSIDONIA. 
Syn. = Posidonomya (Bronn.) 
Gen. Char.—Obliquely oval, slightly convex, usually subtruncate at one end, close to which is a beak-like 
point, about which as a centre the substance is concentrically waved and plicated ; texture extremely thin, semi- 
membranous, flexible, and elastic. 
“Tt seems exceedingly probable, as M. Deshayes has already suggested, that Posidonia may be the internal 
plate of an animal allied to Aplysia. I was particularly struck with the resemblance while dissecting some large 
specimens of the Aplysia depilans of our coast this summer ; the gill-covers of these animals, when observed in 
the recent state, or before being dried, are precisely similar to a Posidonia in shape, and with somewhat similar 
concentric wrinkles, but more faintly marked; their consistence is intermediate between that of thin horn and 
membrane, bending readily, and without injury, in any direction, and capable of receiving very slight impressions, 
but with sufficient elasticity to regain the original shape on the pressure being removed. This seems to 
have been also the state of the Posidonia, as they impress each other in all directions in the shales without 
injury. The fine shales in which they occur must have been deposited in deep water, which is also the habita- 
tion of the recent Aplysiw. The large proportional size of the soft portion of the animal, when compared with 
the small shell, would account for the general dark colour of the Posidonia shales. If the above views were 
correct, the Posidonic should be removed to the Gasteropoda” (Synop. Carb. Foss. Irel. p.77). Since writing the 
above in 1844, I have been strengthened in that view by every subsequent examination both of the fossils and 
the, in my mind, recent analogues; it is, of course, necessary to remove from the true semimembranous Pos¢- 
donice such thick-shelled, undoubted bivalves as the Znoceramus vetustus, &e. which have been confounded with 
them, but which truly belong to the genus Ambonychia (Hall). There is no evidence of the true Posidonia 
being bivalve ; what seem right and left valves being only the two sides of the one plate*. 
PostponiA Becuerti (Lronn.) 
Ref —Gold. Pet. Germ. t. 113. f.6; Sow. Geol. Trans. 2nd Series, Vol. V. t. 52. f. 2, 3, 4. 
Dese.—Obliquely subovate to subrhomboidal; beaks close to the anterior} end, which is very short, rounded, 
with a slight ear-like angulation in front of the beak ; posterior dorsal edge nearly straight, varying from con- 
eave to slightly elevated; posterior end varying from very broadly rounded to obtusely and very obliquely sub- 
truncate; ventral margin moderately convex; surface marked with subregular, obtusely angular concentric 
ridges, varying from two to four in three lines on the middle of the shell, smaller and closer on the anterior and 
yentral portion, largest on the posterior and dorsal half ; abruptly diminishing to small strize, or minute, irregular 
plicze of growth, for a narrow space along the posterior dorsal margin; a minute membranous striation usually 
visible with a lens, parallel to the ridging of the surface. Average length from beak to posterior end two inches 
five lines, proportional width at right angles thereto at middle %, depth *%, length of nearly straight portion of 
dorsal margin on both sides of the beak =. 
* As this sheet was passing through the press Dr Griffith sent me a drawing of a specimen shewing two Posidonicee 
connected by the straight margin like a bivalve; but I fancied I had found in my old dissections of large Aplysice that 
instead of one plate there were often several, placed one over the other and connected by the straight edge which forms the 
posterior margin; this if established, I thought, would explain Dr Griflith’s specimen. A reference to books failed to give 
any information on the matter, but at the last moment I have been fortunate enough to get a specimen from my friend 
Dr Dickie, and a fresh dissection proved the accuracy of my recollection by shewing (although a small young individual) 
two distinct Posidonia-like lamine, connected by the posterior part, or “ hinge” as it would be called by those who believed the 
fossil to be a bivalve. 
} The species are described in the same way as bivalves of similar shape. If the genus be allied to Aplysia, the part 
called “dorsal edge” would be posterior. 
3x2 
