30 



KNOWLEDGE. 



[Febeuary 1, 1901. 



Among tlie fossils to be obtained occasionally from 

 the workmen in large chalk-pits are teeth of the type 

 shown in Fig. 7 ; the specimen depicted having been 

 purchased by myself in a pit near the road between 

 Chatham and Maidstone. As many others were offered 

 at the same time, it doubtless foi-med part of a more or 

 less complete millstone, which, as is too often the case, 

 was broken up by the workmen. These teeth form 

 convex quadrangular bosses, the marginal portion of 

 which consists of a broad granular area, while the 

 centre is occupied by a variable number of bold ridges, 

 or folds, between which are often irregular knobs. It 

 is from these ridges that the fish take the name of 

 Ptychoclus. For a long time it was uncertain how these 

 teeth were arranged, but careful comparison of a number 

 of more or less incomplete series in situ has at length 

 solved the problem. In the lower jaw the complete 

 millstone was formed by a median row of large teeth 

 similar to the one figured, ou each side of which were 

 six or seven other rows composed of teeth gradually 

 decreasing in size from the centre to the margin. In 

 the upper jaw, on the other hand, there was a central 

 row of small teeth, flanked on each side by a row of 

 large ones, externally to which came a series of rows 

 gradually diminishing in size. From this mode of 

 arrangement it is inferred that Ptychoclus was a ray ; 

 and the whole dental structure is as remarkable for its 

 perfection as a crushing machine as it is for its inta-insic 

 beauty. 



Even more elegant from an sesthetic point of view 

 are the " millstones " of the Port Jackson shark 

 {Gestracion) and its allies j the upper jaw of the Aus 

 tralian species being shown in Fig. 6. In place of 

 forming a continuous plate across the palate after the 

 fashion of the eagle-rays, the individual teeth in this 

 gi'oup are arranged in obliciue bands round the edges 

 and inner sides of the jaws,* showing in the hinder 

 region a melon-shaped swelling of remarkable graceful- 

 ness, which would form an attractive ornament for 

 the capital of a jiillar. In this melon-like portion 



Fig. 6. — Upper Dentition of the Port Jatksou Shark (Cesfracion). 

 Fi&. 7. — A Tooth of the Ridge-toothed Ray {Ptychodus). 



of the millstone the individual teeth form blvmtly 

 convex oblongs ; those of one row being markedly larger 

 than all the rest, while the rows in front of and behind 

 this do not correspond with one another in size. 



* Strictly speaking, the tooth-bearing cartilages of sharks are not 

 true jaws. 



Examined with a lens, each of these blunt teeth is seen 

 to have a minutely pitted structure, while its median 

 longitudinal line is marked by a narrow smooth streak. 

 New teeth are being continuously produced on the 

 margin of the series on the inner side of the jaw, and 

 as the outer ones become worn away, the whole series 

 is pushed over towards the edge of the jaw. Proceeding 

 from the larger rows of teeth towards the front of the 

 jaw, it will be seen that as the individual teeth become 

 gradually shorter their smooth median line gains promi- 

 nence, till it finally develops into the sharply pointed 

 cusp surmounting each of the front teeth. 



As already said, the Port Jackson shark and a few 

 other nearly related species (all of which, by the way, 

 feed on shell-fish and crustaceans) are the only sharks 

 with millstones met with in our present seas. And it is 

 fortunate that these have lived on, as otherwise w3 

 should never have gained a true idea of the dental 

 armature of their extinct relatives which abounded in 

 the seas of the Jui'assic epoch. Visitors to "Whitbv 

 must be familiar with certain black oblong fossils of 

 about an inch and a half iu length known to the 

 quarrymen as '' fossil leaches." These are the hinder 

 teeth of an extinct shark [Asteracanthus) nearly allied 

 to the Port Jackson sjaecies, but of much larger size ; 

 and although they are more rugose than pitted, they 

 show the same smooth line on the summit. A beautiful 

 specimen from Caen, in the British Museum, shows that 

 the arrangement of these hinder teeth was almost 

 exactly the same as in Cesfracion, which may thus be 

 regarded as a survivor from a long past epoch of the 

 earth's histoi-y. 



But there were other " millstone-mouthed ' sharks at 

 a still eai-lier period which appear to have been allied 

 to Cestracion, although the degree of relationship is 

 uncertain. In these Palaeozoic sharks, as exemplified 

 by Cochliodus, of which the imperfect millstones are 

 shown in Fig. 2, the series of hinder teeth seem to have 

 had an arrangement very similar to that obtaining in 

 Cestracion, but the individual teeth of several series were 

 more or less completely fused into a single solid plate, the 

 ridges on which mark the original lines of division 

 between the component series. These sharks exhibit, 

 therefore, one among many instances where the earlier 

 forms of a group are in some respects more sjiecia'ised 

 than their descendants. 



So much space has been taken u]) by the rays and 

 sharks that only a few lines remain for the millstones 

 of the enamel-scaled fishes. In none of these do the 

 teeth, which are developed on most of the bones of both 

 the upper and lower jaws, ever form continuous plates ; 

 and they are generally either spherical (Fig. 5) or 

 kidney-beau-shaped (Fig. 3), and an-anged in more or 

 less distinct longitudinal rows. Unlike those of the 

 sharks and rays, these teeth, as in the familiar 

 Lepidotus of the Wealden (Fig. S), are replaced by 

 vertical successors ; and their mode of development is 

 so pectiliar that in some cases the new tooth is placed 

 wrong way up beneath the one it is destined to replace. 

 In other instances, as in Ccslodus (Fig. 3), from the 

 Folkestone Gault, successional teeth have not been ob- 

 served, and the mode of renewal is consequently still 

 unknown. Although within the limits of a single articl3 

 in Knowledge it is impossible to do more than give tli3 

 crudest sketch of a vast subject, yet what has been 

 written may be sufficient to attract my readers' interest 

 to an extremelv fascinating branch of zoological studv. 



