184 J. W. DAVIS ON FISH-SPINES FROM 
four or five, and these gradually converge towards the apex : in the 
opposite direction the number is increased by bifurcation to ten or 
even twelve. The striew or coste are covered with smooth black 
enamel, which towards the base is produced into minute beak-like 
or knotty prominences. The intercostal spaces are more fibrous in 
appearance, and are as nearly as possible the same width as the coste. 
The most anterior ridge is separated by a wider intercostal space 
than the others, and is produced so as to form a carina, or keel, ex- 
tending along the whole length of the exposed part of the spine. 
The posterior portion appears to be excavated only a short distance 
from the base towards the apex; and the remaining portion is pro- 
tected by seven recurved, pointed denticles, long in proportion to the 
size of the spine, and widely separated. The basal or implanted 
part of the spine is conical in form and presents the usual fibrous 
structure. 
The smaller spines (fig. 12) are in many respects similar to those 
already described; they have the same graceful ornamentation of 
the lateral and anterior faces, and the posterior surface has a row of 
pointed denticles. They differ, however, in being smaller, rarely 
exceeding an inch in length, and are less than proportionately broad. 
Their curvature is very considerable compared with that of the larger 
ones; they are almost sickle-shaped; and from the middle portion 
of the spine the edges converge rapidly to the apex, which forms 
an acute point. 
After a careful comparison of the two forms with examples of 
recent fishes, the supposition appears reasonable that the fish to 
which they were attached may haye had two dorsal fins, the larger 
and straighter spines pertaining to the anterior one, and the shorter 
curved spine to the posterior dorsal fin. A very similar arrangement 
may be seen in the existing Spenaw acanthias, in the Cretaceous 
Drepanephorus canaliculatus described by Sir Philip Egerton in the 
13th Decade of the Geological Survey of the United Kingdom, pl. 9, 
and in Palwospinax priscus, 13th Decade, pl. 7. 
The new genus Hoplonchus is most closely allied to the genera 
Homacanthus, Leptacanthus, and Onchus of Agassiz*, and to a new 
genus recently described in the ‘ Memoirs of the American Geological 
Survey of Illinois’ by Messrs. Orestes St. John and A. H. Worthen, 
and named by them Acondylacanthus. From Onchus tenwistriatus, 
which is described by Agassiz as straight or feebly arched, with 
smooth and uniform longitudinal ridges, and having the base bevelled 
to a point, Hoplonchus differs in having recurved denticles at in- 
tervals along the posterior face. 
The genus Leptacanthus is described as long and narrow, with 
very numerous fine longitudinal strie, and also “two rows of closely 
set minute denticles along the posterior margin. ‘The striz do not 
form decided furrows; and in L. semistriatus there is a small space 
free of ridges along each side near the posterior row of denticles. 
The species known to Agassiz were from the Jurassic and Liassic 
* Recherches sur les poissons fossiles, par Louis Agassiz. Monographie des 
poissons fossiles du yieux grés rouge, par L. Agassiz. 
