46 PISCES. 



symphysial teeth are few and relatively large. The hyomandibular is 

 slender, but there is no facette on the pterygo-quadrate for direct 

 articulation with the cranium. Five pairs of branchial arches have 

 been counted, the hindermost very delicate. The notochord must have 

 been persistent, but the arches are calcined (fig. 34, n, k), and there are 

 no traces of intercalary cartilages (both Liassic and Wealden specimens 

 observed). The dorsal fin-spines (fig. 34, d 1 , d' 2 ) are longitudinally ridged 

 and grooved, and bear two longitudinal series of denticles on the hinder 

 face, these being arranged close together mesially. The caudal, anal, and 

 paired fins are unknown. The shagreen is sparse and variable in develop- 

 ment, consisting of small conical, radiately-grooved tubercles, sometimes 

 fused into groups of three. Two large hook-shaped, semi-barbed dermal 

 spines occur immediately above and behind each orbit in some indi- 

 viduals, probably males (fig. 35). Typical remains of Hybodus range 

 from the Muschelkalk (Middle Trias) to the Wealden inclusive, and some 

 species may be even of later date. The best-known species are H. dela- 

 bechei (Lower Lias), H. mediits (Lower Lias), H. hauffianus (Upper Lias), 

 and H. basanus (Wealden). A closely allied genus with blunter teeth 

 is named Acrodus and seems to have a still wider range, the latest teeth 

 of this form occurring in the Gault. 



Palaeospinax. A small fish with dentition closely resembling that 

 of Hybodus, but the symphysial teeth comparatively high-crowned and 

 prehensile. The vertebrae are simple constricted cylinders ("cyclospon- 

 dylic"), sometimes feebly asterospondylic. The dorsal fin-spines are 

 smooth and enamelled, without posterior denticles. The shagreen is 

 fine and dense, but there are no enlarged head-spines ; large hooks occur 

 on the claspers of the male. The typical species, P. prisons, occurs in 

 the Lower Lias, represented by nearly complete specimens ; and there is 

 evidence of other species both in the Rhsetic of England and the Upper 

 Lias of Wurteniberg. Synechodus, from the Cretaceous, is a closely allied 

 genus with well-defined asterospondylic vertebrae, but with a nearly 

 similar dentition (fig. 36). 



Asteracanthus (fig. 37). Teeth (originally named Strophodus} massive 

 and not cuspidate but rounded or keeled, and all superficially ornamented 

 by reticulate markings ; those of the symphysis few, relatively large, much 

 arched, and without lateral denticles. The dorsal fin-spines resemble 

 those of Hybodus, except that the superficial ridges are subdivided into 

 tubercles; the large hook-shaped head-spines are also very similar in 

 these two genera. The notochord must have been persistent. This is 

 a Jurassic and Wealden genus, the typical and best-known species, A. 

 ornatissimus, being especially characteristic of the Upper Jurassic. 



The palaeontology of the Scylliidae presents very few features 

 of interest. One genus (Palceoscyllium] closely related to the 

 existing Scyllium, is known from the Lithographic Stone (Upper 



