Shield-bearing Crustacea. 327 



With the appendages of Hemiaspis we are unacquainted ; 

 but the genera Pterugotus, Slimonia, and Enrypterus, are so 

 well preserved to us that we are able to restore them in the 

 most perfect manner from actual specimens. 



Their appendages offer striking analogies to the Limuli, but 

 at the same time we cannot neglect to notice their strong* 

 resemblance to the scorpions among the Arachnida. 



But these fossil genera seem to have been truly marine, 

 and to have performed their respiration by means of branchiae, 

 as in the recent king-crabs ; the scorpions, on the contrary, 

 are furnished with tracheae for the purpose of aerian respira- 

 tion, as in insects, a most important distinction. 



We are led to regard this order as a more generalized type 

 of Crustacea indicating, in Silurian times, if not representing, 

 the Arachnida of the present day. 



In Limulus we are introduced to a more compact form 

 of organism in which the body segments are concentrated into 

 two parts, the cephalo- thorax and abdomen ■ and lowly as the 

 king-crabs appear in comparison to the Decapods (crabs and 

 lobsters), yet they must be looked upon as a more exalted type 

 than the Eurypterus, and its fossil allies, according to the 

 laws of cephalization, as propounded by Dr. Dana and other 

 carcinologists. 



The " King-crab," or "Moluccan crab" (Limulus), is one 

 of the most persistent types of organisms we are acquainted 

 with higher than the Mollusca. For we cannot, I think, 

 doubt that the king-crab of our modern seas is descended from 

 those of the Jurassic formations of Germany, although none 

 are found in intervening rocks. 



Had Limulus represented a higher type, it is hardly con- 

 ceivable that it could have existed so long, and apparently 

 unchanged. But it seems to belong to one of those eccentric 

 groups that appear from time to time in the zoological series, 

 which, branching out into a by-way of its own, is checked 

 from further onward progress • but being possessed of tenacity 

 of life and great powers of reproduction, holds its ground, 

 whilst higher orders are being modified or swept away. 



That the king-crabs present analogies with the Trilobites 

 few will doubt, especially when we compare them with the 

 Carboniferous species, such as Limulus (?) trilobitoides (Fig. 6), a 

 remarkably trilobe'd form. 



Mr. Salter, in his Monograph on the Trilobites (Palaeon- 

 tographical Society, 1864, page 8), writes: — •'''Every author 

 who has written on Trilobites has more or less perceived their 

 analogy with the Limulus, or king-crab, to which there is, 

 indeed, a good deal of external resemblance. But this resem- 

 blance totally fails when we examine the under-side of the 



