GOLDFUSS ON ORTHACANTHUS. 23 



genus. This is situated at the extremity of the head, and no trace 

 appears hehind it of the rays of a dorsal fiii, of which it might have 

 formed the commencement. The root which was buried in the flesh 

 is not renewed (verjiingt sich nicht)^ but the whole spine gradually 

 increases in thickness from the point to the basis, is quite straight, 

 cylindrical and smooth on the anterior surface, whereas the spines of 

 the Hybodus are compressed, curved backwards, striated on the root 

 and furrowed on the lateral faces. In order to observe the characters 

 of the posterior surface of the spine which was hid in the stone, a 

 portion of it was dug out and an impression taken of the uninjured 

 furrow in which it lay. This showed an elevated central line, with 

 minute points at alternate intervals on each side of it, indicating small 

 spines. These are not placed close to the central line, but at some 

 distance from it on the margins*, whereas in all the known Hybodus' 

 spines, they approximate so closely towards the upper extremity as 

 to form only a single row. There is also no indication of a second 

 spine at the commencement of a posterior dorsal fin, as seems to have 

 been the case in the Hybodus. On the other hand, the spine of this 

 fish has the most perfect similarity to a large spine from the coal 

 formation of Manchester, which Agassiz (iii. tab. 45. fig. 7-9) has 

 distinguished generically under the name of Orthacanthus cylindricus. 

 It must therefore be conjoined with the genus Orthacanthus^ but 

 forms a distinct species of scarcely half the size, characterized by a 

 spine of one-third the thickness and by the distance of the smaller 

 teeth from the middle line. 



The discovery of this fish not only enriches the fauna of the coal 

 formation with a new species, but completes our knowledge of the 

 genus Orthacanthus, of which the following are the characters : — 



Mouth large, corresponding to the circumference of the head, with 

 several rows of small, numerous, three-pointed, slightly compressed, 

 longitudinally furrowed teeth. 



Instead of the first dorsal Jin, a single, straight, flattened, cylin- 

 drical, subulate {platter ^ drehrunder 'pfriemenformiger) spine, which 

 has on the posterior side, at a little distance to the right and left of 

 a raised middle line, a row of fine spines. 



The second dorsal fin, opposite the ventral fin, without a spine. 



Pectoral fins lateral, large, with a long, articulated carpal ray di- 

 rected backwards, and furnished with fin-rays on its inner and outer 

 sides. 



Ventral fins moderately large, with a similar tarsal ray, which, 

 however, has fin-rays only on the outer side. — [J.N.] 



2. Memoir on the Test of Trilobites and on some accidental Distor- 

 tions of its Form ; with Notes on some Species from Brittany. 

 By M. Marie Rouadlt. 



[Bull. Soc. Geol. France, 2nd series, vol. vi. p. 67.] 



The information in this paper may be considered supplementary to 

 the author's former communication on the " Silurian Fossils of Brit- 



* See fig, 11. pi. .), of the original. 



D 2 



