SLIMONIA ACUMINATA. 



109 



The surface of all the segments is much crumpled and creased, clearly indicating that 

 the covering of the body was far less robust than in Pterygotus anglicus, Agassiz. 



The Telson or tail-plate (20) is a large, somewhat oval plate, terminated by a long 

 slender finely pointed spine, often nearly as long as the telson itself. The length without 

 the spine, in fig. 1, PI. XVII, is three inches, and, including the spine, 5| inches. The 

 greatest breadth is 2\ inches; the breadth at its articulation with the last segment 

 1 1 inch. The lower side of the telson is somewhat hollowed, whilst upon the dorsal 

 surface it is marked by a strong median keel which is continued down the centre of the 

 tail- spine to its extremity. 



The margin of this tail-joint is ornamented with a serrate or tuberculate ornamentation 

 near the base (like that on the margin of the head), which is often seen to be continued 

 down the sides of the terminal spine. 



We have already referred to the ocelli and compound eyes which occupy the dorsal 

 aspect of the head-shield ; and must now describe those appendages which are grouped 

 around the mouth on the ventral aspect of the body. 



The Antenna (PL XVII, figs. 1, 2, 2, and 3 ; PL XVIII, figs. 1, 2, 2). This pair of 

 appendages is very small ; and, from the fact of their always lying closely folded down on 

 either side of the mouth, they had remained unnoticed by previous observers until pointed 

 out by me in an account of this genus published in 1863. 1 



The subjoined woodcut (fig. 31) shows the usual position in which they are found. In 



Fig. 31. — Antenna of Slimonia acuminata, Salter, sp. (Natural size). 



PL XVII, fig. 3, the palpus is turned in the opposite direction with regard to its basal 

 or attached portion (endognath) to that in which it usually occurs. 



1 See 'Intellectual Observer,' vol. iv, 1863, pp. 229—237 ; woodcut k, p. 235. 



