PTERICHTHYS PRODITCTITS. 103 



The proportions of the ventral surface of the carapace are variable. In the 

 specimen represented in PI. XXII, Fig. 3, which, according to the contour of 

 the pectoral limb, seems to belong to this species, the shape is like that of the 

 " latus " form of PL Milleri, the length of the anterior and posterior ventro- 

 lateral plates being equal, and the breadth at the median ventral plate being even 

 greater than the distance between the centre of that element and the posterior 

 margin of the carapace. But in the four type specimens figured by Agassiz in 

 PL V of his ' Fishes of the Old Red Sandstone,' we see a tendency for the 

 carapace to become proportionally longer and narrower, and, in fact, it is not 

 possible to differentiate the original of Fig. 2 from the " oblongus " form as 

 represented m his figs. 1 and 2 on PI. Ill of the same monograph. 1 



PI. XXI, Fig. 1, of the present work represents a very pretty specimen of 

 this " oblongus " form, though the sides appear perhaps a little more curved than is 

 usual. Here the posterior ventro-lateral plates are longer by nearly one sixth than 

 the anterior ones, and the breadth of the ventral surface of the carapace at the 

 median ventral plate is equal to the distance between the posterior margin of that 

 dement and the hinder extremities of the posterior ventro-lateral plates. 



Another specimen of this form is seen in Fig. 2 of the same plate, and shows 

 the ventral plates from their internal surfaces, except the left anterior ventro- 

 lateral, which is wanting, as is also the greater part of the median ventral, though 

 the lozenge-shaped space which it occupied is very apparent. This specimen is a 

 typical example of the so-called " oblongus," as is seen by the narrow shape of the 

 ventral surface of the carapace, the proportional length of the posterior ventro- 

 laterals, and the expanded contour of the distal segment of the pectoral limb. The 

 tail, minus the caudal fin, is well preserved, the scales being arranged in longitudinal 

 rows, as in Pt. Milleri, and showing also the same form. An unusually good view 

 of the dorsal fin (d.) is also here afforded, whence it is evident that not only are 

 certain elongated scales placed along its anterior margin, but also that a few of a 

 rounded form clothe the lower part of its sides ; the minute scales covering the rest 

 of the expanse of the fin exhibit, to a certain extent, a linear arrangement. 



The elevated form of the carapace is seen nowhere better than in the specimen 

 from Cromarty represented in Fig. 3 of PL XXI. It is an example of the 

 " oblongus " form, showing a natural cast of the interior of the lateral plates in an 

 absolutely uncompressed condition, and illustrating well the statement of Sir P. 

 Egerton that the " contour of Pterichihys must have considerable resemblance to a 

 high-backed tortoise." It would have looked still more like a high-backed tortoise 

 had the median dorsal plates been present. The space occupied by the posterior 

 dorso-lateral plate {p. d. I.) is worthy of notice on account of the imprints of three 



1 Of the specimens figured on the above quoted plate, the original of Fig. 1 and the counterpart of 

 Fig. 2 are in the British Museum ; the others are in the Gordon Cumming Collection at Forres. 



