THE OLD EED SANDSTONE.

Ill

near the middle of the belly. There is a curious mechanismof shoulder-bone involved with a lateral spine and with thepectorals. The creature, unlike the Clieiracanthus , seems tohave been furnished with jaws of bone; there are fragmentsof bone upon the head, tubercled apparently on the outersurface ; and minute cylinders of carbonate of lime runningalong all the larger bones, where we find them accidentallylaid open, show that they were formed on internal bases ofcartilage. But the best-marked characteristic of the crearture is furnished by the spines of its fins, which are of sin-gular beauty. Each spine resembles a bundle of rods, orrather, like a Gothic column, the sculptured semblance of abundle of rods, which finely diminish towards a point, sharpand tapering as that of a rush. (See Plate VIII., fig. 4.)*The rest of the fin presents the appearance of a mere scalymembrane, and no part of the internal skeleton appears.Perhaps this last circumstance, common to all the ichthyolitesof the formation, if we except the families of the Coccostemand Pterichthys, may throw some light on the apparently mem-branous condition of fin peculiar to the families of this order.What appears in the fossil a mere scaly membrane attachedto a single spine of bone may have had in the living animala cartilaginous framework, like the fins of the dog-fish andthorn-back, that are amply furnished with rays of cartilage,—though, of course, all such rays must have disappeared inthe stone, like the rest of the internal skeleton. Unquestion-ably the caudal fin of the two last-described fossils must havebeen strengthened by some such internal framework; for,as they differ from the other fins in being unprovided withosseous spines, they would have formed, without an internalskeleton, mere pendulous attachments, altogether unfitted toserve the purpose of instruments of motion. There may be

* Agassiz reckons four species of DiplacantTma, — D. crassitpinws, D.lfmyispin.ua. I), atriatulus, and D. striatua.