386 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 16, 



2. Description of the Pterygotus problematicus, Agass. 

 By J. W. Salter, Esq., F.G.S. 



[Plate XXL] 



The limbs of this interesting Silurian fossil not having been hitherto 

 discovered, the present specimen is of considerable interest, as con- 

 necting it satisfactorily with the species so fully figured by Agassiz, 

 which was obtained from the basement-beds of the Old Red Sand- 

 stone of Forfarshire*. But, though of the same genus with the 

 Scotch fossil, it presents characters which separate it specifically. 



As there are on the same slab with it numerous specimens of 

 Avicula retroflexa, Hisinger, with Orthis lunata and Orhicula ru- 

 gata, there can be no doubt of its being in the Upper Ludlow rock ; 

 and we are therefore justified in considering it as belonging to the 

 species which is so common (always in a fragmentary state) in the 

 same stratum throughout Shropshire, Herefordshire, and theMalverns ; 

 and a fragment of the carapace of which is figured by Agassiz, Sil. 

 Syst. pi. 4. f. 4, 5. 



Of the two fragments on the slab, the best preserved is that of a 

 finger (probably the fixed one), PI. XXI. fig. 1. It is 2\ inches 

 long and 7\ lines broad, exclusive of the spines, and of equal breadth 

 throughout ; but it neither shows the base nor the tip, and must have 

 been considerably longer. The substance is very thin, sections of the 

 broken, ends showing this condition very plainly. 



The spines along the inner edge are long, conical, or almost cylin- 

 drical, and are set fully their own breadth apart : they are of various 

 sizes, several small ones being interposed between the larger spines. 

 There are seventeen of the smaller spines on the fragment, each about 

 a line high ; there are two larger ones about ^ inch in length, and 

 one large spine towards the middle, which is broken, but its base is 

 \ inch broad. All the spines turn a little backward (as in P. an- 

 glicus, Ag.) and are finely striated lengthwise, rather obliquely. The 

 surface of the finger itself is very ill preserved and crushed ; it appears 

 not to have been quite smooth, and there are scattered small tuber- 

 cles toward the inner edge, as well as minute prickles interspersed 

 between the spines. 



In P. anglicus, as figured by Agassiz, the spines are much larger 

 and more coarsely striate, and they stand so close together that their 

 bases often touch ; they are, too, fewer in number than in our fossil, 

 and the finger is shorter than this appears to have been. 



The other fragment, fig. 2, is more doubtful ; it has spines along 

 its margin like the last, but they are much larger (about 4 lines long), 

 and more closely placed, so that their bases approach each other. 

 There are also small prickles interspersed as in the last. But the 

 margin on which they are set, instead of being slightly concave, is 

 considerably convex, and, unless it has been much curved by pressure, 

 could hardly have belonged to the extremity of the limb. On this 

 fragment, but probably not connected with it, there is a large conical 

 spine or articulation, constricted at its base ; it is an inch long by 



* Poiss. Foss. du Vieiix Gres Rouge, p. xix. note, and pi. A. 



