NATURAL SCIENCES OF PHILADELPHIA. 267 



is evident he intended it to include only those patelliform palaeozoic shells 

 with the posterior side more or less truncated. Mr. Billings, however, and 

 some others extend it so as to include circular or oval species, showing no 

 traces of the posterior truncation, such as were referred by Phillips and others 

 to Patella. Although it is probable the typical truncated and the oval or cir- 

 cular species without the posterior truncation represent two distinct genera, 

 it is perhaps impracticable in our present state of knowledge to separate these 

 groups, owing to the fact that there are so many intermediate forms ; while 

 it is very rarely indeed that we can know anything in regard to the interior 

 of these fossil shells. 



Phillips says nothing respecting the muscular impressions of his typical 

 species, but his figure of M. obloriga, which seems to represent an internal cast, 

 shows apparently a horse-shoe shaped scar, like that seen in Capulus, Hip- 

 ponyx, and the all'ed genera. Prof, de Koninck has also shown ( Sup. An. Fos., 

 pi. lviii, fig. 1 and 2) this scar very clearly in M. pileus of Phillips, and M. 

 Solaris, = (Patella Solaris, de Kon.) From these figures it is evident, as ob- 

 served by Prof, de Koninck, that the open end of the horse-shoe shaped scar is 

 directed away from the truncated side of the shell, showing that the truncated 

 side is the posterior instead of the anterior, as supposed by Phillips. 



Metoptoma. (Platycekas?,) ujibella, M. & W. 



Shell much depressed or patelliform, circular in outline ; apex central or 

 very nearly so ; sides sloping about equally, with generally a slight concavity, 

 in all directions ; surface marked by fine lines and obscure wrinkles of growth. 

 Muscular scar on each side, elongate-oval and somewhat arched downwards, 

 with a narrower band connecting them behind. 



Length and breadth each about 1*70 iych ; height about 0-70 inch. 



Although not an uncommon shell, we have never seen a specimen of this 

 species with the apex entire, though insome of the casts it looks as if it may have 

 been suddenly projecting and possibly curved. Hence, we are in doubt 

 whether it may not fall more properly within the genus Platyceras, though it 

 is much more depressed and expanded than any species of that genus known 

 to us. As a general thing, the specimens are regularly circular or slightly 

 oval, and without traces of the peculiar truncation of the typical forms of 

 Metoptoma, though some of them seem to show obscure indications of it iu the 

 slightly less prominent outline of the margin on one side. 



On one single partly-worn specimen, apparently agreeing in other respects 

 with the others, there are indications of small, irregular radiating costaa on 

 the lower half of apparently the anterior side. This may possibly be a dis- 

 tinct species, but we cannot be sure of this without more specimens for com- 

 parison, since the typical specimens are mostly internal casts. 



Prof. Winched has described, from the Kinderhook beds at Burlington, 

 Iowa (Proceed. Acad. Nat. Sci., Phila., July, 1865), a somewhat similar 

 species, but judging from its measurements, it must be distinctly less depressed 

 than our shell, and differs in being " contracted at the aperture." 



Locality ami position. Burlington division of the Subcarboniferous series, 

 Quincy, Illinois ; also in same position on Honey Creek, Henderson county, 

 Illinois. 



Po*LYPHEMOPSIS CHEYSALLIS, M. & W. 



Shell subfusiform ; spire conical, moderately elevated, pointed at the apex ; 

 volutions nine, a little convex and increasing gradually in size, last one form- 

 ing about two-thirds the entire length and moderately produced below ; suture 

 distinct ; aperture narrow suboval, acutely angular above and narrowly 

 effuse below ; inner lip apparently wanting ; columella a little arched and 

 twisted ; surface showing only very faint traces of lines of growth. 



Length, 0-55 inch; breadth, 0*23 inch; apical angle convex on the slopes, 

 divergence about 40 Q . 



>6] 



