FISHES OF THE UPPER LUDLOW ROCK. 



607 



" Figs. 40 to 45 represent rays of the fins, belonging probably to the genus Onchus, and which 

 appear also to indicate another new species. Figs. 37, 38 and 39 do not appear to me deter- 

 minable at present ; perhaps they may be rays, or perhaps tubercles of the skin ?" 



ICHTIIYODORULITES OF THE UPPER LUDLOW ROCK. 



1. Onchus Murchisoni, Agass. (PL 4. f. 9, 10, and 11.) 



" The defensive fin," observes M. Agassiz, " which I have termed Onchus Murchisoni, is very 

 characteristic. Its lengthened, slender, and almost straight form, its imperceptible tapering away, 

 the great size of the longitudinal ribs, and the depth of the alternating furrows, are traits which 

 distinguish it at a first glance from every other species of this genus. It is very interesting (adds 

 M. Agassiz) that your Upper Silurian Rocks should have produced a species so remarkable, and of 

 the distinct nature of which no doubt can be entertained. The presence of three of these defences, 

 in one small specimen which you sent me, would seem to indicate, that the species is not very rare. 

 It is, however, to be observed, that the fishes to which they belonged, bore a spine upon each of 

 the two dorsal fins. At all events this is the case with the living genera {Cestracion, Centrina and 

 Spinax), with which it must be compared, though such affinity may be rather distant." 



2. Onchus tenuistriatus, Agass. (PL 4. f. 57, 58, and 59.) 



" The above-named figures best represent this species, while figs. 12 and 13 show the cavity which 

 occurs in the posterior side of the base, and which is more or less prolonged into the interior of the 

 spines f rayons' of all the cartilaginous fishes with bony dorsal spines 1 . This species differs from 

 the preceding, both in having finer and closer ribs, and in its general form, which is more arched. 

 The rays are also proportionally shorter than those of Onchus Murchisoni." Extracts from letters 

 of M. Agassiz to myself. See also Rdcherches, torn. iii. p. 6, since published. 



Coprolites. 



The coprolites formerly alluded to as occurring so abundantly in this " bone bed" are represented 

 in the figures 46, 47, 54, 55. Having submitted them to Dr. Prout, whose analysis established the 

 views of Dr. Buckland concerning the faecal origin of similar bodies in the lias and other formations, 

 that distinguished chemist has favoured me with the following account of those which occur in the 

 hone bed of the Ludlow rocks. 



" I find all the specimens examined to consist essentially of the phosphate of lime and of the car- 

 bonate of lime. There was, however, a considerable residuum undissolved by acids, consisting ap- 

 parently of silex, oxide of iron, together with carbonaceous and other matters, the nature of which 

 was not ascertained. 



1 For an account of the proof that the dorsal spines of cartilaginous fishes have a bony structure, see struc- 

 ture of ichthyodorulites in preceding Chapter. 



4 h 2 



