268 



HARD WICKE'S SCIENCE-GOSSIP. 



of recurved booklets on the hinder border. It much 

 resembles the spine of Hybodus (fig. 165), and differs 

 chiefly in the denticulation of the longitudinal ridges. 

 The teeth included in Agassiz' genus, Cladodiis 

 (fig. 162), consist of a central principal cone, with 

 several lateral cones, and arc particularly remarkable 

 from the circumstance, that the outermost of the lateral 

 cones are the largest. 



The Carboniferous spines known as Tristyclihis 

 have been found in association with teeth that cannot 

 be distinguished from those of Hybodus,^ and these 

 detached teeth have, indeed, been previously referred 

 to the familiar Mesozoic genus. The more our 

 knowledge progresses, however, the more difficult 

 does it appear to define these ancient Selachians from 

 the characters of isolated spines and teeth, for 

 modern researches seem to show that spines alone are 

 of very little value in determining affinities, and that 

 some types of dentition arc common to several 

 distinct genera.t 



Carcharopsis and Pristiiladodiis (fig. 161) are two 

 very imperfectly known genera, represented in the 

 Lower Carljoniferous strata by solitary detached teeth, 

 and may possibly belong to Hybodonts, although 

 some palaeontologists are inclined to regard them as 

 indicating sharks of the existing family of Carchariidce. 

 Agassiz, who founded the first genus, pointed out the 

 resemblance of the tooth to that of Carcharodon, and 

 suspected that Carcharopsis and Carcharodon might 

 be closely related to each other, but, after considering 

 all that is at present known concerning the obscure 

 fossils, Mr. J. W. Davis (of HaHfax) has latelyj 

 decided that they may be more correctly placed in 

 the family now under consideration. 



The general characteristics of Hybodits are com- 

 paratively well-known, owing chiefly to the excellent 

 preservation of many examples in the Lower Lias of 

 Lyme Regis ; but w'e are still ignorant of its exact 

 shape, and the peculiarities of the paired, anal, and 

 caudal fins. The body is covered with shagreen, and, 

 as in CtcnacantJius, there are two dorsal fins, each 

 armed with an anterior spine (fig. 165), the first of 

 these powerful weapons of defence being longer, more 

 slender, and more arched than the second. The 

 ceeth vary much in form in the different species, but 

 generally consist of a central principal cone, with one 

 or more secondary cones on each side ; sometimes 

 {H. minor) ihey are remarkably acute, and at other 

 times (//. Delabcchci) blunt ; sometimes (// grossi- 

 conits) the cones are well marked off one from 

 another, while at other times (//. raricostatiis) only 

 slight furrows separate them ; and, in a few instances 

 {H. medins, &rc.), none but a single cone with gently 

 sloping sides can usually be recognised. Tlie first 

 complete mouth of a Hybodus was described liy Sir 



* T. Stock, "Ann. & Mag. Nat. Hist.," Sept. 1883. 

 + '1'. Stock, " Nature," Vol. 27 (1882), p. 22. 

 J "Trans. Roy. Dublin Soc," Ser. 2, Vol. I. (i88j), pp. 

 3S1-J83. 



Philip Egerton, in 1845,* when the nearly perfect 

 skull of a new species [H. basanits) was discovered in 

 the Wealden strata of Pevensey Bay, Sussex. The 

 specimen proved that the teeth of the upper jaw 

 scarcely differed at all from those of the lower, that the 

 dentition varied much less in different parts of the 

 mouth than is the case in recent sharks, and that, 

 therefore, detached teeth almost always admitted of 

 specific determination. These conclusions have been 

 confirmed to a great extent by more recent discoveries 

 in other formations, and there are very few species 

 in which all the teeth are not readily recognisable as 

 variations of a single type-form ; the small IT. 

 Dubrisicnsis, of the Chalk, appears to have the most 

 complex dentition of any of the British Hybodonts, 

 the front teeth lieing markedly acute and prehensile, 

 while the hinder ones are much flattened and 

 elongated. 



One of the most singular features of Hybodus, 

 consists in its possession of curious curved spines on 

 the upper part of the head.t These cephalic spines 

 (fig. 164) were only known to Agassiz in an isolated 

 condition, and were originally described by that 

 eminent ichthyologist as teeth, under the name of 

 Sphcnonchiis. They have been found in association 

 with remains of most of the English species, and it 

 seems probable that they were common to all : it is 

 uncertain at present how their number and arrange- 

 ment varied in each, but H. Ddabcchci,X from the 

 Lower Lias of Lyme Regis had four of these dermal 

 appendages, — a pair on each side of the head, a 

 little above the orbit, — and there are indications of 

 some others possessing the same number. They 

 are generally regarded as being of the same nature as 

 the dermal spinous tubercles so characteristic of 

 some of the Rays. 



As already stated, the earliest remains of Hybodus 

 occur in the Continental Muschelkalk, and the first 

 appearance of the genus in Britain is in the Keuper 

 Series. Teeth and spines (dorsal and cephalic) of //. 

 Kcupcrinus have been found both in Warwickshire 

 and Somersetshire. The English Rhxtics yield //. 

 minor. H. raricosiatus, and some others, and a forma- 

 tion of similar age in Moray, Scotland, contains spines 

 and teeth of //. Lawsoni. The Lias, particularly the 

 lower division of Lyme Regis, is exceedingly rich in 

 remains of Hybodus, and the beautiful specimens 

 obtained from it have afforded more information con- 

 cerning the genus than examples from any other 

 stratum. The principal species are as follows : — H. 

 reticiilatiis (of which the posterior dorsal spine was 

 originally described as //. curtus) with teeth (fig. 166) 

 consisting of sharply-pointed cones, well marked off 

 from each other, and the principal cone much the 

 largest ; //. Delabcchci, with teeth (fig. 167) somewhat 



* " Quart. Joiirn. Geol. Soc," 1845, pp. 197-1O9. ?•• IV. ^ 

 t First pointed out by E. Charlesworth, " Mag. Nat. Hibt., 

 18,9, p. 24s. V\. IV. 

 X E. C. H. Day, -'Geol. Mag.," 1865, p. 565- 



