DAVIDSON — SCOrnSU CARBONIFEROUS DRACHIOrODA. 
109 
Var. A, Pml/er/its semirefiniliitiis, figs. 1-8, is trnnsvcrsoly oval, or of a 
rouiulccl quadrate sliaiio, hut it is also at times somewhat (-lougatod, tlic hiiige- 
liue heiiig either rat licr sliorter or as long as the greatest width of the sliell. Tlic 
ventral valve is idwajs gibhous, witli and sometimes without a shallow longitu- 
dinal median sinus, or depression, the auriculate cardinal expansions being 
moderately developed but clearly defined ; the beak is wide and incurved, both 
usually eovcrtid with irregular concentric wrinkles, always larger and deeper 
upon the auriculate expansions, and the eut-ii-e surface of the shell is covered with 
numerous radiating longitudinal rounded striffi, from which project at variable 
int ervals tabular spines of moderat e length. The width of the strise, as well as 
the interspaces between them, varies also according to the specimen, two or 
more usually occupying the breadth of a Ime. The larger number are simple, 
but others bifurcate here and there, and sometimes two or more (in rare cases) 
will unite towards the margin, so as to form but a single rib, whUe others are 
due to intercalation. Several ribs will also at times cluster togetlier, so as to 
produce an elevation, and thus giving the frontal portion of the shell a some- 
what grooved appearance. The spines are likewise more numerous, larger and 
longer in certain examples than in others, but always most so upon the auriculate 
portions of the vfdves. The dorsal valve is slightly concave, or flattened to 
some distance from the hinge, so that a considerable space was left free for the 
soft portions of the animal ; the external scidptm-e is also very similar to that 
which has been described for the opposite valve. The ventral valve is thicker 
than the dorsal one ; both become extremely thin and sometimes recurved 
near their margin. The interior of both valves have been often procured, and 
in excellent preservation, as may be seen from the tigures of our plate, and we 
will merely notice that in the ventral valve the occlusor impressions are situated 
almost upon a level with the divaricator scars, and much lower in the valve 
than for example in P. giganteiis, etc. In the dorsal valve the occlusor impres- 
sions are often beautifully sculptm-ed, and the cardinal process is trilobed. 
Pro(li(ctus semiredculatus has sometimes attained large dimensions, a Scottish 
specimen represented in our plate has measured tw'o inches three lines in length 
by two and a-half inches in width, and it has elsewhere assumed still larger 
proportions. P. sulcattts appears to be nothing more than a smaller variety of 
the same, wherein the median sulcus of the ventral valve is more than usually 
deepened, and upon the lateral portions of the beak (close to the aiu-iculate ex- 
pansions) there existed sometimes, but not always, a somewhat elevated ridge, 
with a row of rather large prominent spines. P. Scoticus appears to me to be 
undoubtedly a variation of shape only of the species under description, and not 
of P. giganteus, as has been supposed by some palaeontologists. I have had the 
loan of the original figured specimens for several months in my possession, and 
both valves will be found represented in our plate. 
Yar. B, or Martini, figs. 10-12, is distinguished from the preceding one by 
the great length and irregularity of its anterior prolongation ; the arched beak 
is suddenly bent downwards in an ahnost straight line, giving to some speci- 
mens a peculiarly elongated and genieulated appearance. The dorsal valve is 
slightly flattened to some distance from the hinge-line, when it closely follows 
the cui'ves of the opposite one. The thinness of the shell sometimes makes it 
liable to fracture at some distance from the beak, as may be seen in one of the 
figures ; the lateral portions of the valve are likewise much dilated, with 
numerous spines sometimes projecting from the auriculate portions of the 
valve. The beak is concentrically wrinkled, and the entire sui-face is covered 
with thread-like strise, which bifurcate sometimes several times, especially upon 
the lateral portions of the shell. 
This is the variety to which Martin in 1809 applied the specific denomination 
of Anomites produdus, and of which Sowerby's P. coiiciimus is evidently only a 
