249 
Jn the visceral region, vaulted, geniculate, or reflected on itself; surface convexly 
rounded, no sinus ; sides vortical, or even sometimes slightly concave separating the 
remainder of the valve from the more or less short, flattened, triangular ears ; umhonal 
region often depressed from above, sometimes greatly recurved, and sometimes more 
or less incurved over the hinge-line ; the latter is of median length, but a little less than 
the width of the valve, sharp and acute ; front usually much recurved and occasionally 
laterally expanded, assisting to give the shell much of its geniculate appearance ; beak 
small and acute ; surface, when the outer shell is preserved, shining and silky, with a few 
growth constrictions towards the front, and indistinct undulations on the umboual 
region ; longitudinally and indistinctly eostated by delicate, elongate, parallel spine 
bases, within the substance of the shelly matter, penetrating the latter, as small, free, 
short, tubercle-like spines, giving a general prickly or pimply appearance to the entire 
ralve, especially towards the front. When the thin outer shell is removed, or in 
decorticated examples, these long spine bases leave a series of fine channels on the 
surface of the fossil. Ears with numerous tubular spines. The cardinal muscular 
Scars are but little marked ; the adductors are straight, and formed of oblique ramifi- 
cations ; suiface granular. 
The dorsal valve varies in the amount of its concavity, being at times shallow, 
at others exceedingly concave, especially in the middle lino ; ears flattened ; front 
produced ; surface with concentric lines and spine bases. The septum is moderately 
short ; dendritic adductor impressions small ; reniform impressions laterally extended, 
and well marked ; internal surface granular. 
Ohs. Froductus hraeliylhcems is one of the most interesting species of the genus 
yet published, and has been much misunderstood by several of those who have written 
about it. The species is well worth a separate and detailed study. Chance observers 
examining the various figures hitherto published would be apt to consider them as not 
all appertaining to one species. If full attention, however, is given to the various 
aspects under which the shell is found, I believe it can be shown that they one and all 
represent a single well-marked and peculiar species. 
All who have written on P. hrachytlimrus lay stress on the shortness of the hinge- 
line as compared with the width of the front. This, with the elongated decurrent bases 
of the spines, forming channels in the shell, are particularly characteristic points in 
B. Sowerby’s species. The channelling of the shell by the spine bases is seen in 
many species of Prodttctm, but it appears to be peculiarly distinctive of P. hmcliytdiosrus. 
libe length of the hinge-line varies according to age, in large and old individuals it 
becomes longer, and the shell, which is very convex and geniculate, loses some of its 
convexity and also widens out. 
In describing this species the late Professor John Morris referred to it two shells 
of very different aspect and state of preservation. One of these is a cast in sandstone, 
showing the general form, and more particularly the channels formed by the decurrent 
ases of the spines. The other specimen is a decorticated siliceous cast, and displays 
no internal characters of both valves to great perfection. On the ventral valve are 
exposed the node-like prominences of the cardinal muscles, the scars of the more 
emngated adductor muscles, and the internal cast of the beak. The dorsal valve shows 
he scars of the adductors, and the cast of the septum, which in this individual reaches 
^ most to the front margin of the valve, an ususual length in the genus Productus. 
ee vascular impressions are also preserved and come very far forward, like the 
®®ptum. 
In his Work on the “ Genus ProduciuSy' again in that on “ Productus and Clwnetes" 
^^d more recently in his “Eossiles Paleozoiques de laNouvelle-Gallesdu Sud,” Prof. De 
