THE BONE-BED AT AUST, NEAR BEISTOL. 



417 



lateral surfaces are ornamented with strongly marked ridges of 

 enamel, which anastomose frequently, and present a somewhat 

 wavy outline on account of the enamel forming the ridges not 

 being of uniform thickness, in some places presenting a beaded 

 appearance. The ribs lie roughly parallel with the anterior surface 

 of the spine, and run out along the posterior edge, but not in a 

 very well defined manner. The junction of the ribs with the basal 

 portion is not so well denned as in H. reticulatus : in the latter 

 the ends of the ribs rise above the surface of the base • whilst in 

 the Aust specimen the ridges blend with the fibrous structure of 

 the base, and the hollows between the ridges are below the level 

 of the basal surface. The anterior and lateral portions of the spine 

 have a circular or, rather, dome-shaped form in section. The 

 posterior forms a wide base, not flat, but a little produced outwards 

 towards the centre. The whole of the posterior portion higher 

 than the open part of the cavity retains the fibrous structure of 

 the base ; and along each side of the median portion extends a row 

 of blunt, laterally compressed, enamelled denticles ; they are about 

 •2 inch across the base, and rise "1 inch from the surface of the 

 spine. Each denticle is separated from the next by a distance a 

 little greater than its own diameter. 



These spines differ from those of H. reticulatus in several particu- 

 lars : their form is stronger and more robust ; the lateral surfaces are 

 rounder, and the base and cavity wider. In comparison with its 

 width, the spine is shorter, not so gradually pointed, and less curved. 

 The posterior denticles are not pointed and recurved towards the base, 

 as they are in H. reticulatus; nor do its characters agree with 

 those ascribed by M. Agassiz to H. minor, as already stated ; and it 

 appears necessary that a specific name to indicate this spine should 

 be instituted : I suggest that it be Hybodus austiensis. 



Htbobtjs ptjnctattts, Davis. (Plate XXII. fig. 2.) 



An imperfect spine from the bone-bed possesses characters which 

 remove it from any other species hitherto described. It consists 

 of the upper portion of a small spine 1 inch in length ; the 

 antero-posterior diameter is *2 of an inch at its broadest part ; 

 the transverse diameter is a little less ; the spine is slightly 

 curved, more so on the anterior than the posterior surface, and 

 tapers gradually to a point. The lateral surfaces are covered with 

 longitudinal ridges, separated by grooves of about equal diameter, 

 numbering five on each side. Along the bottom of each groove 

 there are a number of minute pittings extending in a line 

 parallel with the groove. Towards the points the ridges become 

 less prominent and gradually disappear, the apex being quite 

 smooth and without striae. There is a row of denticles along 

 each latero-posterior surface ; they are prominent, obtusely pointed, 

 laterally compressed. The internal prolongation of the pulp-cavity 

 is comparatively small (less than one third the diameter of the 

 spine), situated nearer the posterior than the anterior surface, and 

 conforming in outline with the oval form of the spine. 



