HARDWICKE'S SCIENCE-GOSSIP. 



107 



the jaw than in the typical species of Hybodus, and 

 there are some dissimilarities in microscopical 

 structure. The symphysial teeth approach a conical 

 form, and there is sometimes a slight indication of 

 lateral or secondary cones. It must be remarked, 

 however, that some species, such 'as A. Anningia 

 (fig. 84), are quite on the borders of the two genera, 

 and the ornamentation on a few of even the most 

 characteristic Acrodont teeth (fig. 74) is suggestive of 

 their close relationship to those of the true Hybodont 

 type. 



The dorsal spines of Acrodus, unknown to Agassiz, 

 were first described in the " Geological Magazine,"* 

 twenty years ago, by Mr. E. C. H. Day. In this 

 elaborate paper, he points out how nearly they 

 resemble those of Hybodus, and is unable to discover 

 more than two points of difference between them. 

 He endeavours to show that, in spines of the latter 

 genus, the double row of posterior denticles is fixed 

 upon a somewhat prominent ridge, as seen in the 

 section (fig. 75), while in Acrodus, the back of the 

 spine is comparatively flat (fig. 76) ; also, that the 

 denticles themselves are fewer and stouter in Acrodus, 

 than in Hybodus. But it must be remembered that, 

 since the date of these studies, much more valuable 

 material has accumulated, and it is questionable 

 whether, when a large number of specimens, such as 

 are now available, are examined, many intermediate 

 gradations will not be found. The object of Mr. 

 Day's paper is, indeed, to prove that Hybodus and 

 Acrodus are closely allied, and that the only differ- 

 ences between them are merely in degree and not in 

 kind ; and he concludes a very careful discussion of 

 their characters by suggesting that, according to their 

 dentition, the Hybodonts and Acrodonts might be 

 regarded as forming a single group, divisible into 

 three sections : — " the first, with very elongated 

 cones, represented by H. basanus ; the second, with 

 the cones more obtuse, by H Delabechei ; and the 

 third, almost or altogether wanting conical elevations, 

 by A. nobi/is." How far these conclusions are to be 

 accepted, future research must decide. 



Species of Acrodus range from the Triassic to the 

 Upper Cretaceous strata, inclusive. The Continental 

 Muschelkalk has yielded A. Gaillardoti and others, 

 and the Rhaetic of Devonshire is characterised by the 

 little A. minimus (fig. 77). A. nobi/is (fig. 73) and 

 A. Anningim (fig. 84) are the most important species 

 of the Lias, being found chiefly in the lower divisions, 

 and not so abundantly as the remains of Hybodus. 

 A. hiodus and A. kiopleurus occur in the Stonesfield 

 Slate ; and two species, A. IUingworthi and A. cre- 

 taceus, have been described! from the English chalk. 



The genus Strophodus is not quite so well known 

 as that just considered. No certain information has 

 hitherto been obtained concerning any feature in its 



* " Geol. Mag.," 1864, vol. i. pp. 57-65. 



t Dixon's " Geology of Sussex," 1st edit., 1850, p. 364. 



organisation beyond the dentition,* and only one 

 specimen affording a definite clue to the arrangement 

 of the teeth appears to be yet known to science. 

 This beautiful example is preserved in the Oolitic 

 Caen Stone, and was described by Sir Richard Owen 

 in the "Geological Magazine" for 1869. It 

 exhibits about sixty teeth in situ, and is represented 

 in fig. 72. As regards the arrangement of the 

 different dental forms, it bears a close resemblance to 

 the jaw of Ccstracion, but differs from the living 

 genus in the same respect as does the jaw of Acrodus, 

 namely, in the symphysial teeth being much fewer 

 and relatively larger. There are two principal rows 

 of crushing teeth (fig. 72, a, b), as in Ccstracion, and 

 there are likewise indications of some posterior rows 

 of smaller and somewhat elliptical teeth (ib., c) ; 

 but, instead of nine rows occupying the space in 

 front of the principal series on each side, only three 

 are to be observed (ib., b, c, d), and no median 

 azygous row is present. The teeth themselves, when 

 isolated, are readily distinguished from those of 

 Acrodus by means of their surface-ornament, which 

 consists of reticulate markings, but a glance at the 

 figure of the Caen specimen is sufficient to show the 

 extreme difficulty of determining the species of such 

 detached fossils. 



Strophodus ranges from the Upper Permian to the 

 Chalk, inclusive. It is represented in the Kupfer- 

 schiefer of Germany by S. arcuatus, and at least one 

 species is also found in the Triassic Muschelkalk. 

 S. magnics is characteristic of the Lower Oolites, and 

 other so-called species (S. tenuis, &c.) likewise occur 

 upon the same horizon ; ^. favosus is the name of 

 some small teeth (fig. 78) from Stonesfield. The 

 Middle and Upper Oolites,— particularly the Oxford 

 and Kimmeridge Clays, — yield the well-marked 

 form, 6". reticu/atus (fig. 79), which is easily recog- 

 nised by the prominence of its ornamentation : of 

 this species we know more than any other, except 

 S. medius (fig. 72), a large number of teeth having 

 been found associated in the Kimmeridge Clay of 

 Shotover, and described by Agassiz in his great work 

 on the " Poissons Fossiles." The Cretaceous series 

 contains the last traces of the genus, so far as is yet 

 known, and only two forms appear to have been 

 recorded from this group ; one is S. sulcatus, from 

 the Greensand of Maidstone, and the other the very 

 rare and curious S. asper (fig. So) of the Chalk. 



Ptychodus is an essentially Cretaceous genus, and 

 has not hitherto been met with in rocks of any other 

 age, either in the Old or New World. Nothing 

 beyond the dentition is known with certainty, 

 although Agassiz, in his original description of this 

 shark, associated with the teeth certain peculiar 

 elongated fossils which he thought might be the 



* It has been suggested "that the spines known under the 

 name of Asteracanthus really belong to Strophodus ; but abso- 

 lute proof is at present wanting, and we shall thus reserve their 

 consideration for the chapter on " Ichthyodorulites." 



