THE BONE-BED AT ATJST, NEAR BRISTOL. 



419 



In the species described by M. Agassiz the spines are as nearly as 

 possible twice as much in their antero-posterior as in in their trans- 

 verse diameter. In a specimen from the Bristol Museum (Plate 

 XXII. fig. 3) the spine from back to front has a diameter of '35 inch ; 

 and its width across the posterior surface is -3 inch, or very 

 nearly equal to the antero-posterior diameter; the external pos- 

 terior groove is shallow ; and no denticles or tubercles are present. 

 There is a very large median keel along the front of the spine. It 

 is almost round, but rather wider than deep. It is composed of 

 shining black enamel, and constitutes one fourth of the entire dia- 

 meter of the spine. The spine is imperfect, the apical portion 

 missing ; so that it is impossible to say whether the sides were tuber- 

 culated. It appears to have been less curved than the common 

 forms, the portion preserved being straight. 



A second specimen, more closely resembling N.JiUfer, is a flat-sided 

 spine of the ordinary kind (Plate XXII. fig. 4), finely striated along 

 each lateral face. The anterior keel is small and threadlike ; along 

 the posterior surface are blunt, widely separated denticles. This 

 spine is '25 inch in diameter, and, when perfect, would probably be 

 about 2 inches long. The sides of the spine are covered with 

 longitudinal strise as in the type specimens. 



Both the varieties noticed above are from the collection of Mr.Ord, 

 of Bristol, and were collected from the bone-bed at Aust Cliff. 



Nem acanthus minor, Davis. (Plate XXII. fig. 5.) 



Spine imperfect. Length 1-1 inch, diameter -1 inch, when 

 perfect probably nearly or about 2 inches in length. In section 

 it is circular. A canal or internal cavity of similar form ascends 

 the centre of the spine towards the point (the latter broken off in 

 this specimen). There is no evidence that the cavity was open 

 along the posterior surface ; but it appears to have been terminal. 

 The spine is ■ slightly curved in form. Its surface is slightly and 

 irregularly grooved, and is further ornamented by a number of mi- 

 nute papillse. In the latter respect the spine resembles the genus 

 Nemacanthus : but it differs in other essential respects from either of 

 the two species described by Prof. Agassiz*. It does not exhibit 

 any trace of having a ridge of any kind along the anterior 

 surface ; instead of that, it is round and indiscriminately spotted 

 with papillae or tubercles. The section of the spine is round, and not, 

 as in the species of Agassiz, oval or triangular. It appears proba- 

 ble that the spine may belong to the genus Nemacanthus; but it is 

 quite separated specifically. I propose the name N. minor in allusion 

 to its small size compared with those previously described. 



The specimen figured is in the museum of the Geological Society 

 and is labelled " from the Eucoid bed, Wainlode Cliff," and was 

 presented to the Society by H. E. Strickland, Esq. 



* Poissons Fossiles, vol. iii. p. 26, tab. 7. figs. 9 & 10-15. 



