LEPTOPLASTUS SALTER!. 91 



margin. Fixed cheeks less than the width of the glabella posteriorly, less than 

 half the width at the eye. Free cheeks marginate, with the genal spine springing 

 from the middle of the lateral margin ; in front of the spine the external margin, 

 forms a smooth curve, behind it an oblique straight line meeting the posterior 

 margin at an obtuse angle ; genal spines short, directed outwards and backwards. 



Thorax of twelve segments. Axis wider than the pleurae in the first few 

 segments, about the same width behind, with a iwv of median spines, commonly 

 represented by tubercles on the internal cast. Pleurae bent downwards about 

 half-way between the axial furrow and their terminations, deeply grooved, ending 

 in short curved points. 



Tail small, broad, with a narrow raised margin. Axis wider than the lateral 

 lobes, ending bluntly at the posterior margin, divided into three distinct rings and 

 a terminal portion. Lateral lobes with two furrows on each side. Margin either 

 entire, or sometimes with a minute point at the antero-lateral angles. 



Dimensions. — The most nearly perfect specimens that I have seen are only 

 6 or 7 mm. long; but more fragmentary specimens indicate a length of 15 mm. 

 Raw says that " the adult reaches a length apparently of 1| or 2^ in., but only 

 two specimens indicate such sizes." 



Callaway observed the presence of median tubercles upon the thoracic 

 segments; but even these are not usually distinct except upon external moulds, 

 and internal casts often appear to be perfectly smooth. It is only in exceptional 

 circumstances, when the matrix has split along the median plane, that the spines 

 are clearly shown. They were first noticed by Thomas, who looked upon the 

 spiny form as a distinct species, which he called Olenvs mitehinsoni. It is evident, 

 however, from the specimen figured in Plate XI, fig. 5, and from other similar 

 specimens, that when the test w T as filled with sediment the spines usually remained 

 empty except at their bases. Either the spines were actually solid or the hollow 

 was too fine for the sediment to enter. Consequently, on internal casts there is no 

 indication of the spine except a small tubercle, while in external moulds the 

 impressions of the spines are generally conspicuous, though it is seldom clear that 

 they were anything more than tubercles. 



Since the form described by Thomas cannot be considered distinct, Lejpto- 

 plastus Salter i is the only species of the genus known in Great Britain, and 

 accordingly the generic characters are sufficient to distinguish it from other 

 British trilobites. It presents, however, a close general resemblance to several 

 of the Scandinavian species of Leptoplastus. In L. stenotus, Ang., the axis of the 

 thorax is somewhat narrower proportionately, and it bears neither spines nor 

 tubercles ; according to Angelin the number of segments is eleven, but Persson 1 

 describes and figures twelve ; the tail is similar in shape to that of L. salteri, and 

 1 Geol. F6reii. Stockh. Forh., vol. xxvi (1904), p. 523, pi. ix, figs. 14—16. 



