LAMELLIBRANCHIATA. | UPPER PALAZOZOIC MOLLUSCA. 497 
By examining the lines of growth of old specimens having the narrow, subacuminate, obliquely truncated 
posterior end, with the slight sinus in front of the respiratory angle, it will be noticed that the outline at younger 
stages of growth differed by having a rather wider and less obliquely truncated posterior end, and no sinus in 
front of the respiratory angle, from which we may doubt the distinctions of form on which the Axinus rotundatus 
and Schizodus truncatus of King have been founded, to be really distinctive. Of the S. Schlotheimi of Geinitz 
I forbear to speak, having no specimens ; but the character of greater elongation and obliquity of the posterior 
end may possibly only indicate aged dwarf specimens. I have examined the small specimens from the red Per- 
mian marl of Manchester, described by Capt. Brown, and I have no doubt of their being young and dwarf spe- 
cimens of the species more largely grown in the limestone beds. Prof. King, on examining the original specimen 
of A. undatus of Brown, finds the flexure to be a break in tHe margin, not coinciding with the lines of growth. 
Position and Locality.—Common in the Permian magnesian limestone of Humbleton Hill. 
MYoPHORIA ROTUNDATA (Brown Sp.) 
Ref. and Syn. = Axinus rotundatus Brown, Manch. Geol. Trans. Vol. I. t. 6. f. 29. (? + A. pusillus and Lucina 
minima id. id. fig. 32, 33.) = Schizodus Rossicus M. V. K. Geol. Russ. t. 19. f. 7,8.+ Schizodus truncatus 
King, Perm. Foss. t. 15. f. 25 to 29. (and fig. 30, as S. rotundatus.) 
Desc.—Ovato-rhomboidal, anterior side very broadly rounded; posterior end obliquely subtruncate, broad ; 
hinge-line slightly oblique; ventral margin gently convex, nearly straight towards the respiratory angle; beaks 
moderately large, prominent; diagonal ridge from beak to respiratory angle obtusely rounded ; posterior slope 
slightly concave; valves moderately tumid, most so at a little more than one-third the width from the beaks, 
from whence the surface arches gradually to the margins; posterior half of the shell nearly smooth, marked with 
subregular lines of growth, which rise abruptly on the anterior side, into thread-like elevated lines, nine in two 
lines at six lines from the beak. Teeth and muscular impressions as in the generic character. Length of rather 
small specimen nine lines, in proportion to length greatest width from beak to opposite ventral margin 77, length 
of anterior side ;, width of posterior end about =, length of hinge-line about >, depth of each valve 3. 
The more rounded form, the broader and less obliquely truncated posterior end, and the consequently less 
ovate form, and shorter posterior side, are the characters supposed to separate this species from the J. obscura ; 
the shell is also said to be thinner; but all these are characters shared by the young specimens of J. obscura ; and 
as the size of the present species is less than that of the typical 17. obscwra I am by no means certain that they 
should be considered specifically distinct, and rather incline to the opinion that a larger suite of specimens than 
I have at command may shew their identity. The figures given by Prof. King seem to support this view; some 
of the latter shew the original colouring in small triangular dark blotches. 
Position and Locality —Not common, usually under an inch in length, in the Permian magnesian limestone 
of Humbleton Hill; also in the red Permian marls near Manchester, 
Genus. CLIDOPHORUS. See page 273. 
CiipopHorvs costatus (Brown Sp.) 
Ref. and Syn.= Arca costata Brown, Trans. Manch. Geol. Soc. Vol. I. t. 6. f. 34, 35.= Plewrophorus costatus 
King, Perm. Foss. t. 15. f. 13 to 17.= Cypricardia Murchisoni Geinitz, Grundr, t. 19. f. 2. 
Desc.—Elongate, oblong, or subelliptical; dorsal and ventral margins subparallel, very slightly convex or 
almost straight ; hinge-line nearly as long as the shell; anterior end very short, semielliptically rounded ; pos- 
terior end obtuse, rounded, very slightly oblique; beaks small, very slightly projecting, inclined obliquely, and. 
approximate over the anterior end: casts shew a very strong sulcus of the clavicular ridge, extending from the 
front of the beak along the posterior edge of the large anterior adductor, and the strong cartilage plate running 
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