216 FOSSIL FISHES OF THE ENGLISH CHALK. 



tlie posterior dorsal fin-spine is less arched and somewhat smaller than that just 

 described. Similar fin-spines, found detached, were originally named Sphiax 

 major by Agassiz (184-3), but it is uncertain as to how many of them belong to 

 this species, or even to Cestrncum. Some are doubtless referable to Synechodus. 



A few grannies of shagreen are seen on the snout (PI. XLV, fig. 1, s.). 

 They are all quite smooth, and most of them are quadrangular in shape, with a 

 small but deep, vertically-fluted base of insertion (fig. 1 d). 



Some teeth of Cedradon larger than those of the typical C. canalicnlatns are 

 occasionally found in the lower zones of the Chalk, and seem to be distinguished 

 by their more sigmoidally-bent shape, less rounded ends, and blunter coronal 

 eminence. Two such specimens are shown in PI. XLV, figs. 4, 5. They have 

 been provisionally referred to Cestracion rngo.^u!^, which is known by detached 

 teeth from the Danian of Maastricht, Holland (A. S. Woodward, Catal. Foss. 

 Fishes B. M., pt. i, 1889, p. ;335, pi. xi, fig. 29; and Proc. Yorks. Geol. and 

 Polyt. Soc, vol. xii, 1891, p. 67, pi. ii, fig. 8) ; but their identification with the 

 Danian species is probably incorrect, and it is possible that they merely represent 

 unusually large individuals of the species now described. There may, however, be 

 a second species of Cestracion represented in the English Chalk ; for comparatively 

 stout and blunt anterior teeth are known, in which the coronal surface is rugose at 

 the base {Gomphodns, Reuss : A. S. Woodward, Proc. Geol. Assoc, vol. xiii, 1894, 

 p. 194, pi. V, figs. 9, 10). 



Horizons and Localities. — Turonian zone : Mailing, near Lewes. Zone of 

 Holaster snhglohosus : Southeram and Glynde, Sussex. Undetermined zones : near 

 Bromley, Kent ; Guildford, Surrey. 



Genus SYNECHODUS, A. S. Woodward. 

 Synecliodiis, A. S. Woodward, Proc. Geol. Assoc, vol. x, 1888, p. 288. 



Generic Characters. — Pterygo-quadrate cartilage in the adult directly connected 

 with the cranium by a postorbital articulation. All teeth cuspidate, those at the 

 symphysis few and with more than one pair of lateral denticles, those behind 

 becoming extended and low-crowned, with the principal cusp and lateral denticles 

 often reduced to small tubercles or points. Vertebral centra distinctly astero- 

 spondylic. Dorsal fin-spines probably smooth, enamelled, without posterior 

 denticles. Shagreen fine ; no cephalic spines. 



Type Species. — Synechodus duhrisiensls, from the English Chalk. 



BemarJcs. — This genus is most closely related to the Liassic Palscospinax, from 

 which it differs especially in the liigher degree of calcification of its vertebral 

 centra. The teeth, when found isolated, have most commonly been referred to 

 Hyhodas, sometimes to Acrodns. The dorsal fin-spines have not yet been discovered 



