4 o NATURAL SCIENCE. Jan., 



always exhibit the "arenatus" ornament, and are often much abraded 

 at the free extremity. The paired spines are triangular, resembling 

 those of the Carboniferous Oracanthus, but with a less extensive 

 internal cavity, and these are also often abraded at the tip. They 

 may have been arranged along the lower margin of the body as in 

 certain Acanthodian sharks (e.g., Climatius) ; or the animal may have 

 had only a single pair of these spines at the back of the head, as 

 described by Dr. Traquair in Oracanthus (5). 



Nothing is known of the teeth of the old armoured sharks, but 

 there is reason to believe (5) that some of them at least formed a 

 crushing pavement such as occurs in Cochliodonts (10). Modern 

 researches, however, are tending to show that the nature of the 

 dentition among early Elasmobranchs does not differ so much in 

 different genera and families as has sometimes been supposed. The 

 tooth named Diplodus, for example, does not differ much from that 

 of the existing Chlamydoselache, although these two fishes belong even 

 to distinct orders. Moreover, it is now proved that the tooth named 

 Cladodus is common to more than one kind of Palaeozoic shark. Teeth 

 of this character are merely the result of the fusion of the primitively 

 separate conical cusps into little clusters of varying form and extent, 

 each cluster fixed on a single base. The tendency to fusion seems 

 to have been the same in all the old sharks, and it is thus now time 

 to admit that we can no longer determine Palaeozoic " genera " and 

 "species," or even "families," on the evidence of detached teeth. 

 The earliest and simplest known form of shark's tooth is Protodus jexi 

 from the Lower Devonian of Canada, this exhibiting merely a single 

 cusp on a comparatively large base. The Lower Carboniferous teeth 

 named Dicentrodus are only sometimes equally simple ; and there are, 

 of course, several later sharks in which some of the teeth at least are 

 unicuspid, while the base remains much expanded. 



In short, it may now be definitely stated that the primaeval 

 sharks differed from their modern representatives in the marked 

 tendency to fusion of their hard skin-structures. The shagreen 

 granules merged into great plates of armour. The pointed teeth 

 fused at their bases into small clusters of cusps, such as we know 

 under the names of Diplodus and Cladodus. The obtuse teeth amal- 

 gamated into crushing plates, like those described as Cochliodus, 

 Psephodus, etc. The dermal fin-rays grouped themselves into cut- 

 water spines, as among the Acanthodians. 



Of greatest interest, however, is the internal skeleton of these 

 old sharks. We know very little about it at present, but progress 

 is gradually being made ; and our knowledge of the fins, at any rate, 

 has been much advanced within the past two years. Until the 

 description of the pectoral fin of Cladoselache ("Cladodus") fyleri in 

 Natural Science for March, 1892 (9), the most primitive type of 

 paired fin known was Gegenbaur's " archipterygium " — a paddle 

 with a jointed central axis of cartilage, fringed on either side with 



