EURYPTERUS LAXCEOLATUS. 143 



The central appendages are each divided into three joints, and the entire surface 

 of the thoracic plate (PI. XXVIII, fig. 2 a.) is closely covered with the usual minute 

 scale-like markings peculiar to this group, which are' also observable upon the anterior 

 half of each thoracic segment, the abdominal segments being smooth and free from all 

 ornamentation. The larval eye-spots — so clearly seen in the American species of Eurypterus, 

 and in the British Slimonia and Pteryyotus — can scarcely be discerned in E. lanceolatus, 

 but they are present, nevertheless, although but feebly indicated. 



There is, without doubt, a close affinity between the genera Eurypterus and Slimonia. 

 The latter approaches the former and recedes from Pteryyotus proper, not only in the 

 absence of chelate appendages, but also in the more narrow and elongated form of the 

 abdominal segments, and the more lanceolate form of the telson, which in Pteryyotus 

 proper is broader, less acute, or even bilobed. The chelate antenuules in Pteryyotus are 

 not adapted for palpi, but in both Eurypterus and Slimonia the basal joints of the simple 

 antennae perform that office. 



Eurypterus differs, however, from both Pteryyotus and Slimonia in the position of the 

 eyes, the form of the carapace and the telson, and, lastly, in the structure of the thoracic 

 plate. 



Dimensions of E. lanceolatus. — The largest and smallest specimens known are from 

 Mr. Robert Slimon's collection and are now preserved in the British [Museum, the 

 former (PI. XXVIII, fig. 1) measuring 1\ inches in length, and 2 inches across its fourth 

 and widest thoracic segment; the latter (PI. XXVIII, fig. 3.), measuring only IS lines in 

 length by 4 lines across its widest segment. A specimen belonging to Mr. Wright, and 

 examined by me in 1S64, measures Cf inches in length, but is much distorted and com- 

 pressed laterally. The length of the specimen (PI. XXVIII, fig. 2) in the British [Museum 

 is 4^ inches, of which the head forms f of an inch, the six thoracic segments 1 inch, the 

 six abdominal segments 1 \ inch, and the telson 10 lines. The fourth thoracic segment is 

 the widest, and measures 1 inch across ; the succeeding segments taper gradually to the 

 ninth, which is scarcely 7 lines in width ; and at the twelfth segment the abdomen is 

 only f of an inch wide. The first eight segments are of nearly equal depth, or about \ of 

 an inch; the ninth, tenth, and eleventh segments are about f of an inch in depth, and 

 the twelfth more than f of an inch deep. The swimming-feet reach down to the fifth 

 thoracic segment. 



The large specimen of E. lanceolatus (PL XXVIII, fig. 1) measures 7 inches in 

 greatest length and If inch across the widest body-segment. The head-shield is 

 \\ inch in length and 1 inch 7 lines in greatest breadth. The measurements of the body- 

 rings are as follows : 



