HARDWICKE'S SCIENCE-GOSSIP. 



227 



18S3.* These teeth are of the same general type as 

 those of Petalodus, but differ in several respects, and 

 are notably longer and narrower, with the crown 

 more spatulate. When one of them became useless 

 in the mouth of the fish, and its successor was ready 

 to come forward for active service, the old tooth did 

 not fall out, but was always retained beneath the new 

 one as a support; as the creature approached old 

 age, the tooth in use had thus a considerable series 

 of worn-out predecessors beneath it, and these seem 

 finally to have become more or less anchylosed 

 together. Such series are not unfrequently found in 

 the Armagh limestone, and one consisting of five 

 teeth is represented in fig. 156 ; the lower and 

 smallest tooth evidently indicates a young stage of 

 the creature's existence, and as the mouth enlarged 

 so did the dentition. The fact that some of these 

 rows are symmetrical, while others appear "lefts" 

 and "rights," suggests that they were originally 

 .ranged alongside each other ; and there is reason to 

 believe that one median tooth was present, with 

 three on each side, but absolute proof is yet wanting 

 of more than one pair occurring besides the median. 



Still more interesting and satisfactory are the 

 .remains that have been discovered of the genus 

 Janassa. This is typically a Permian form, often 

 met with in the Kupferschiefer and the English Marl- 

 slate, but the researches of Messrs. Hancock, Atthey, 

 and Barkas have revealed numerous beautiful 

 examples in the Coal Measures of Northumberland 

 and Durham, and Mr. John Ward has also recorded 

 a few scattered relics from North Staffordshire. It 

 ought to be remarked, however, that the Carboni- 

 ferous forms were originally described under a 

 distinct generic name, Climaxodus, and are often 

 -quoted thus ; but there seems to be no doubt as to 

 'their identity with Janassa, and Miinster's Dictcea 

 is now likewise considered synonymous. Each tooth 

 consists both of a cutting edge (fig. 158, a), and a 

 crushing surface {ib., b), and, like other Petalodonts, 

 possesses a well-developed root {ib., c). The dental 

 -armature of the mouth consisted of five of these 

 teeth, ranged side by side, and flanked by a pair 

 (fig. 157) that are indistinguishable from Petalodus, 

 except perhaps in their obliquity ; this arrangement 

 is shown in fig. 157, taken from an elaborate memoir 

 by Messrs. Hancock and Howse in the "Ann. & 

 Mag. Nat. Hist." for 1869 (vol. iv. ser. 4). The 

 vertical disposition of the teeth and their mode of 

 succession is also known, and the same palaeonto- 

 logists published the illustrative diagram copied in 

 fig. 158, from which it is obvious that, as.in Petalorhyn- 

 chus, the successive new teeth must have arisen from 

 behind, and, on coming forwards, not caused the old 

 ones to fall out, but have rested upon and utilised 

 them as a support. 



Taking into account these various well-ascertained 



* In this work will be found a full account of the Petalodon- 

 .tida?, with references to previous literature. 



facts, we arrive at the conclusion, that although the 

 teeth themselves are more like those of sharks than rays, 

 their arrangement in the mouth agrees most closely with 

 the dentition of such typical rays as Myliobatis and 

 Zygobatis, and the Petalodonts must thus be looked 

 upon as probably intermediate forms. Something 

 like a transition from cutting teeth to crushing teeth 

 may even be noticed in the family itself, for Petalodus 

 (Carb. Limst.) is exclusively laniary, Petalorhynckus 

 (Carb. Limst. and Yoredale) makes a slight approach 

 towards the development of a tritoral portion, the 

 so-called Climaxodus (chiefly Coal Measures) is 

 adapted for both purposes, and the front cutting edge 

 in some specimens of the Permian Janassa becomes 

 almost obscured. 



The curious teeth known as Ctenoplychius (fig. 1 59) 

 and Harpacodus, also, most probably belong to the 

 Petalodontidae, and are both Carboniferous genera, 

 the former ranging throughout all divisions, and the 

 latter being exclusively confined to the lower. No 

 definite evidence of their mode of disposition in the 

 mouth has yet been obtained, and it ought to be 

 remarked that certain small club-shaped fossils, 

 originally referred to a species called C. icuilateralis, 

 are most likely not teeth at all, but Labyrinthodont 

 scutes.* 



PRISTIOPHORID/E. 



According to Dr. Giinther, this small family is 

 only represented at the present day by species 

 of the genus, Pristiophorus, which exist off Austra- 

 lian and Japanese coasts ; these are little Selachians, 

 with the snout much prolonged as in the next family, 

 the " Saw-fishes," but having the gill-openings 

 lateral. Fossil forms are rare, and the only im- 

 portant genus usually referred here is the remarkable 

 Squaloraja of the Lias. A partly restored sketch of 

 this fish is given in fig. 160, and among its many 

 peculiarities may be specially noted the cephalic 

 spine {ib., s), first described by Mr. William Davies, 

 of the British Museum.f When a complete specimen 

 is met with, this spine is generally so compressed and 

 bent down upon the snout as to be rendered incon- 

 spicuous, but it is sometimes found detached, and 

 occasionally (probably in females) it appears to be 

 absent. The vertebrae are usually nothing more than 

 calcified rings, and parts of the body are provided 

 with dermal tubercles. 



Pristid^e. 



The " Saw-fishes " constitute a small family of 

 rays, chiefly inhabiting tropical seas at the present 

 day, and seem to have left no undoubted traces of 

 their past existence in strata of an earlier period than 

 the Eocene. They are particularly remarkable for 



* T. Stock, "Ann. and Mag. Nat. Hist." [5], vol. viii. 1881, 

 pp. 90-95. 

 t " Geol. Mag.," vol. ix. (1872), pp. 145-150, PI. IV. 



