942 



CLASS PISCES. 



to the beautiful preservation of many of the specimens from the 

 Lias, in which deposits these fishes were abundant. Many of the 

 numerous species attained very large dimensions ; and the genus 

 may be denned by the following characters. The teeth (fig. 865) 

 are conical or cuspidate, the crown being more or less striated, with 



one main cone, flanked by one or 

 more lateral cones, and the root 

 being more or less depressed. 

 The teeth at the symphysis are 

 large and few in number ; while 

 the fin-spines (fig. 866) are lon- 

 gitudinally grooved and ridged, 

 with two rows of denticles placed 

 near the posterior borders. The most remarkable feature is, how- 

 ever, the presence of two large hook-like spines immediately behind 

 each orbit, which have been described under the name of Sphen- 

 onchus. The notochord is persistent. 



Fig. 867. — Tooth of Acrodus nobilis, from 

 the Lower Lias of Lyme-Regis. 



Specimens of the Lower Liassic H. Delabechei show the whole of the 

 dentition in situ ; and it appears from these that there was no median 

 symphysial row of mandibular teeth, but that there were ten transverse 

 rows of lower teeth, with five teeth in each row, while the number of rows 

 in the upper jaw was either nine or ten. In the later forms, such as H. 



basajiiis of the Wealden, the teeth 

 differ from those of the typical 

 Liassic forms by the taller, com- 

 pressed, and nearly smooth crowns, 

 and Mr Woodward suggests that 

 it may eventually be advisable to 

 refer these types to a distinct 

 genus. The orbital hooks found 

 in the Oxford and Kimeridge clay 

 indicate fishes of very large di- 

 mensions. 



The genus Acrodus is closely 

 allied to the preceding ; but 

 the teeth (fig. 867) are non- 

 cuspidate and more rounded. 

 This genus is abundant in the 

 Lias, where it is represented by 

 the large A. nobilis, which is the 

 type ; but it also extends down- 

 wards to the Muschelkalk, and ranges upwards as high as the Chalk. 

 The majority of the species are European, but the genus has also 

 been recorded from the Cretaceous of North America, as well as 

 from certain beds in that country which have been stated to be 

 Miocene. 



Fig. 868. — Part of the palate oi Aster acanthus 

 ornatissimus, from the Great Oolite of Caen. 

 Reduced. (After Owen.) 



