2 BRITISH FOSSILS. 



close to the hinder margin of each cheek, leaving the edge elevated as 

 a thickened and very narrow rim ; and then, at the angles of the head, 

 making a slight curve, runs into and furrows the base of a long spine. 

 This spine is of a subulate form, and projects backwards in a curved 

 manner from the head angles, and at a distance equal to rather less than 

 the breadth of the cheek, bends backwards and straight to more than the 

 tail's length beyond the posterior extremity. No trace of a facial suture 

 has been observed in any of the specimens as yet discovered. 



The axis of the thorax is convex, and nearly equal in breadth 

 throughout, very slightly narrowing at its junction with the tail. Its 

 width is about two-thirds of that of the pleurae. Each joint of the axis 

 is deeply furrowed centrally, elevated anteriorly and posteriorly, the 

 elevations of the proximate joints uniting to form a strong convex, 

 somewhat curved, rounded rib, running across the axis, and terminated 

 at each side by a conspicuous tubercle. Six such ribs mark as many 

 thoracic articulations. The axis is separated from the pleura? on each 

 side by a deep but narrow groove. The pleurae are straight, narrow, 

 and depressed, tumid along their lengths on each side of a conspicuous 

 deep oblique furrow ; extremities obliquely truncated, slightly turned 

 backwards. The surface of the tumid portions of the thorax, and also 

 of the more convex parts of head and tail, are minutely granulated. 



Tail triangular, with the sides slightly curved outwardly. Its axis 

 prominent, gently tapering for nearly two-thirds of its length, then 

 becoming parallel-sided, and ending obtusely at the termination of the 

 tail. Nine ribs like those crossing the thoracic axis, but smaller and 

 without marginal tubercles, cross the tapering portion of the axis ; the 

 remainder is ringed by numerous minute ribs. The axis of the tail is 

 divided by a deeply-impressed furrow from the sides, which are gently 

 convex, and grooved by eight strongly-marked lateral furrows, oblique, 

 and curved slightly backward ; the interspaces are tumid. The upper- 

 most furrow commences at the junction of the tail with the body, and 

 proceeds obliquely onwards to the margin, so as to leave a narrow, 

 lanceolate, triangular space in front of it. The distance between its 

 marginal extremity and that of the next furrow is greater than the dis- 

 tance between the extremities of any of the other furrows. The last 

 and shortest furrow originates just above the termination of the tapering 

 part of the tail-axis, and between it and the extremity is a smooth and 

 slightly tumid triangular space. The margin of the tail has an elevated 

 rim, the deflected portion of which is striated. 



Young specimens have the same number of thoracic joints with adult 

 examples, and do not appear to differ materially in any respect. 



The length, exclusive of the frontal spine, of the largest specimen ex- 

 amined, was one inch and three twelfths. The frontal spine measured 



