﻿PTERYGOTUS BILOBUS. 57 



At first sight it would appear that the body of Pt. bilobus, var. a, was destitute of the 

 characteristic markings discernible upon the larger examples of this genus ; but such is 

 really not the case, for although not always readily seen, they can, with the aid of a good 

 pocket-lens, usually be detected upon some part of the body- a 



segments. It consists in this, as in the larger, species of numerous 

 small semicircular plicae (the curve opening forwards, see Woodcut), 

 which probably covered the anterior half of each segment, leaving 

 the posterior destitute of ornament (see PL XIII, fig. \<f). FlG ^ oT narnen t 



The head in this as in the other species is destitute of of p/ ilfoTas tS 



SCUlptUre On itS SUrfaCe. a. Anterior border. 



We must now proceed to the examination of the under side of the head and the 

 appendages. 



Numerous specimens belonging to this species can now be seen and studied, both 

 in the Museum of Practical Geology, Jermyn Street, and in the British Museum, in 

 which many of the appendages still remain in situ ; whilst in other examples they are 

 found displaced, but preserved upon the same slab with the entire body ; from which it is 

 evident they have become detached by the quiet dissolution of the softer muscular tissues 

 of the animal. 



In the accompanyingWoodcut (p. 58) Ihave delineated, of the natural size, the oral append- 

 ages belonging to the entire specimen oiPterygotus bilobus, var. a, drawn upon PL X, fig. 3. 

 These (by some slight movement in the fine, soft, clayey sediment in which the animal was 

 enveloped) must have become separated from the rest of the body before decomposition 

 had proceeded very far ; for, although fractured and imperfect in detail (by reason of the 

 present indurated character of the matrix), they still clearly prove that the entire buccal 

 apparatus — held together by its integument — has thus been preserved to us. 



The separate organs are represented as they lie upon the surface of the same slab of 

 shale in which the entire body (PL X, fig. 3) is preserved. Their correct order and 

 position may be readily understood by turning to Part I, PL VIII, fig. 1, where I have 

 given a restored figure of the under side of the great Pt. anglicus, with its oral appendages 

 in place. The antenna (Woodcut, fig. 10, l, l ; PL X, figs. 1, 2, a, a) are about 2\ inches 

 in length by \ inch in breadth at the base of the chelae, the moveable ramus being about 8 

 lines long. Both the fixed and the moveable rami of the chelae are slender ; the former is 

 slightly shorter than the latter ; their points are sharply incurved ; the inner borders are 

 armed with minute teeth, which, like that of the larger species, vary in size, being at 

 intervals marked by one much larger than the rest. 



There appear to be five joints in the antennae of this species, but it is seldom that their 

 true line of articulation can be readily distinguished. Pive are clearly to be seen in one 

 of the antennae figured in the accompanying woodcut, fig. 10. 



