﻿PTERYGOTUS ANGLICUS. 43 



except near its proximal or attached end, but it is ornamented with rough, blunt 

 squamae and serrations along the lateral margins. 



The telson figured on PL VI is the largest known, and measures nine inches in 

 length by seven inches in breadth. 



In describing the divisions of the body in Pterygotus anglicus, we have spoken of the 

 head and the body, avoiding the terms " thoracic" and " abdominal somites" as much as 

 possible. On reference to the restored figure at Plate VIII, fig. 1, it will be seen that 

 there is evidence, according to the paired appendages (see ante, p. 6), of 7 somites having 

 coalesced to form the cephalic division. 



Assuming that all the segments united to form the head are represented by pairs 

 of appendages, we have then a true Cephalon ; the 7 succeeding free somites are thoracic, 

 the abdomen being represented by 5 somites 1 and a telson, or terminal joint. But 

 assuming the antennules to be wanting (see ante, p. 37), we must conclude the head to 

 represent 8 coalesced segments, that is to say, 7 cephalic and 1 thoracic, the latter 

 bearing the " operculum," or "thoracic plate" (PI. VII. fig. 1 op). 



In this case onlv the first six anterior somites will be counted as thoracic, the 

 posterior six will be reckoned as abdominal, and the 'telson' will make up the 21 

 segments. 



The likeness which Pterygotus offers to Limulus is very much strengthened by this 

 latter view. As we proceed to the illustration of the other genera belonging to this 

 order, we shall be still more strongly impressed with the many points of close resemblance 

 which they will be found to exhibit. 



The Merostomata seem, in fact, to present us, in these early times, with a parallel 

 group of Crustacea to that embraced in the order Decapoda at the present day ; the 

 Macroura being represented by Pterygotus and its allies, the Brachyura by Limulus, &c, 

 and the intermediate forms, like Hemiaspis, &c, being comparable to the Anomoura. 



We could scarcely, in the whole Zoological Kingdom, point to a stronger illustration 

 of the persistence of type than is to be observed in these Palaeozoic forms. 



Prom our present knowledge of the almost perfect remains of Pterygotus anglicus, 

 and on the evidence of the numerous detached portions of this extinct genus, we are 

 justified in concluding that it attained a length of six feet, and a breadth of nearly 

 two feet at the widest part of its body. 



Prom its large eyes, its powerful natatory appendages, and from the general form of 

 its body, we may also infer that it was a very active animal. Its great prehensile 

 antennae and rows of serrated jaws, further indicate its predacious habits, whilst from its 

 size it must have been the shark of the Devonian seas. 



"No existing Crustacean," observes Prof. Huxley, "has so massive a body as 

 Pterygotus^ . . . . " but mass in an active animal involves large muscles, and 



See Dr. Milne-Edwards' observations on arrested development, ante, p. 6. 



