25:2 DESCRIPTION OF THE SPECIES OF PTERYGOTUS 
what on the great tooth, and more direct in the smaller ones. In 
P. gigas and P. ludensts they are coarser, and the teeth broad. The 
great terminal mucro is as broad and long as the primary tooth, or 
even longer, and is bent at right angles to the shaft. 
Swimming Feet, Basal Joint, Plate XII. [Plate 23] figs. 11-14.— 
Several fragments have been found of the great basal joint, and one 
tolerably perfect from the Whitcliffe, Ludlow, fig. 11. It shows a 
wide-expanded basal lobe, and the whole extent of the serrate tip, 
with the usual number of teeth (thirteen, or rather twelve in this 
specimen) ; and in fig. 14, the upper tooth being obsolete. Fig. 13 a, 
from Ludlow, shows the full number. 
The lobe in front of the teeth is arched and thickened in all the 
specimens, (a character in which this species differs widely from 
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Fig, 12 
Pterygolus problematicus, Ag., thoracic or abdominal appendage? nat. size. Comus Wood, 
Ludlow, in Upper Ludlow Rock (Cabinet of Mr. J. Harley, King’s Coll.) 
P. anglicus,) and the second tooth is more than twice the breadth 
of any of the others, conical, and but little curved; the remaining 
teeth are long, straight, narrow, and separated by about their width 
from each other in large specimens. These elongate teeth are very 
distinctive of the species. I believe I am not mistaken in referring 
Agassiz’s figure of the Sphagodus tooth to this portion. 
etastoma (Plate XII. [Plate 23] fig. 15, Plate XIV. [Plate 25] 
fic. 19, is cordate-ovate, narrower at the bilobed end, and has its 
greatest width below the middle (our figure does not express this 
well), becoming angulated at that point. The notch is deep, the 
lobes rounded (the base is broken off, surface marked with open 
