106 BRITISH FOSSIL CRUSTACEA. 



tongue-shaped median plate (7, c) in situ, covering the first and second thoracic rings. 

 The six broad thoracic segments, followed by six narrow abdominal somites, the last 

 four of which are almost quadrangular in form, and terminated by a finely pointed spear- 

 shaped tail-plate or telson, contrast markedly with the more robust and compact body in 

 Pterygotus, with its rounded head-shield and great chelate antennae. 



The Head-shield (PI. XVIII, fig. 2 ; PI. XIX, fig. 1) in general form is oblong, the sides 

 slightly swelling for about two thirds of the length, and contracting about the anterior 

 third to again expand slightly at the anterior angles, where the prominent oval eyes are 

 placed (o, o) ; the front border is somewhat rounded and, as well as the sides, crenated, or 

 rather tuberculated along the edge. 



The posterior border is incurved, and the lateral angles rounded. A kind of double 

 tuberculate border is conspicuous down the sides of the head, the tubercles being elon- 

 gated, while on the arched front border there are three or four rows. Two short 

 minute keels, one on either side of and close to the median line, mark the hinder border 

 of the head, and are also repeated on the posterior edge of each of the first six body- 

 segments. 



The Ocelli. — The two larval eye-spots (ocelli,/) are distinctly to be seen near the 

 centre of the head-shield, and about one third of its length from the front border. 



The Compound Eyes. — The facets composing the great compound marginal eyes can be 

 not unfrequently clearly seen with a hand magnifying-glass. 



Mr. Erxleben has endeavoured to give on PI. XIX, fig. 2, a representation of the 

 compound eye in Slimonia, magnified four times, but the facets are drawn upon rather too 

 large a scale. They measure with the micrometer one tenth of a millimetre each. 



Head-shields of Slimonia are very numerous both in the British Museum collection and 

 in the Museum of Practical Geology, Jermyn Street. 



That on PI. XIX, fig. 1, measures 5f inches in greatest breadth and 6f inches in 

 greatest length. That on PI. XVIII, fig. 2, is 2f inches in breadth and 3£ inches in 

 length. 



The head of Slimonia was very well known to Mr. Salter at the time of the publica- 

 tion of the ' Geological Survey Monograph/ as appears from the figures given on pi. ii 

 accompanying his description, and from a woodcut in the text at p. 03. Mr. Salter gives 

 an outline of a head measuring G|- inches in breadth and 1\ inches in length from the 

 anterior to the posterior border. He does not notice the larval eye-spots on any of the 

 Eurypterida; they have, however, been observed in Eurypterus remipes by Professor 

 James Hall, in America. 1 



No ornamentation of any kind has been detected upon the general surface of the 

 head-shield. 



1 ' Nat. Hist.,' New York, Part iv ; ' Palaeontology,' vol. iii, plates 80—84, pp. 3S2— 419. 



