90 BRITISH CAMBRIAN TRILOBITES. 



hinder part of the thorax, they are produced into long spines. In Leptoplastus the 

 number of thoracic segments is 12 (according to Angelin 11 or 12), in Eurycare it 

 is 12 — 17; while in Sphserophthalrnus and Gtenopyge there are apparently only 9 

 or 10. 



Frequently the head alone is available, and in such cases it is sometimes very 

 difficult to determine to which of the two groups the specimen should be referred. 

 As a rule, however, in Leptoplastus and Eurycare the glabella is shorter and 

 more nearly square in outline, while in Sphaerophthalmus and Gtenopyge it is 

 somewhat more elongated and more rounded in front. 



Leptoplastus differs from Eurycare chiefly in its proportions. In both genera 

 the head is wide in comparison with its length, but in Eurycare the width is greatly 

 exaggerated. The extra width lies mainly in the fixed cheeks, and consequently 

 in Eurycare the eyes are more remote from the glabella than in Leptoplastus. In 

 Eurycare, moreover, the genal spines are longer and the thoracic axis is narrower 

 relatively to the pleurae. 



Accordingly the following are the principal distinguishing features of the genus 

 Leptoplastus : 



As in the other members of the group the genal spines spring abruptly from the 

 middle of the external margin of the cheeks. The head is wide, but not so wide as 

 in Gtenopyge and Eurycare. The glabella is nearly square. The eyes are set 

 approximately in the middle of the cheeks. The fixed cheeks at the eye are 

 narrower than the glabella. The genal spines are short. The thorax consists of 

 twelve segments, the pleura? ending in short points. The tail is small, and its 

 margin is either entire or is furnished with rudimentary points. 



1. Leptoplastus salteri (Callaway). Plate XT, figs. 2— 5. 



1874. Conocoryphe Salteri, Callaway, Quaint. Journ. Geol. Soc, vol. xxx, p. 196. 



1877. Ohnus Salteri, Callaway, ibid., vol. xxxiii, p. (566, pi. xxiv, fig. 5. 



1900. Olenus Mitch in no ni, H. H. Thomas, ibid., vol. lvi, p, 619, pi. xxxv, figs. 5 and 6. 



1907. Leptoplastides Salteri, Raw, Rep. Brit. As^oc, 1907, p 513. 



Head transverse, width more than twice the length, straight in front, with a 

 marginal rim and furrow. Glabella about four-fifths as wide as it is long, narrow- 

 ing slightly forwards, rounded in front; three pairs of oblique glabellar furrows, of 

 which the first is very short and often indistinct ; neck-furrow well-defined, neck- 

 segment with a median tubercle or spine. Eyes distant from the glabella, less than 

 half the width of the latter, nearer to the anterior than to the posterior margin ; a 

 faint ocular ridge, nearly at right angles to the axial line, running towards the 

 anterior angles of the glabella. Facial suture probably inframarginal in front, 

 crossing the margin a little outside the glabella, in front of the eye forming a 

 curve convex outwards, behind the eye running obliquely outwards to the posterior 



