424 Beviews — Dr. 0. Jaekel — Armoured Palceozoic Sharks. 



before it is possible to proceed. When the author remarks that 

 unsymmetrical spines never occur in the Selachii, we would inquire 

 in what essential characters the cephalic spines of Hyhodus, Acrodus, 

 and Asteracanthus differ from Erismacanthus, Gampsacanthus, and the 

 slender paired spines of Menaspis. Furthermore, the spine named 

 Myriacanthiis yranulatus has never been regarded as a head-spine 

 (" Kopfstachel "), as Dr. Jaekel states : and it is another error to 

 assert that the type-species of the so-called Prognathodus has been 

 in part claimed as pertaining to the same fish as Myriacanthus granu- 

 latus. To suppose that the typical Cochliodont dentition is of an 

 essentially different character ("wesentlich anderes Gebiss " ) from 

 that of all true Selachii, displays a very superficial acquaintance 

 with existing knowledge of the subject ; and when the author sug- 

 gests that this dentition may have been firmly anchylosed with the 

 supporting cartilages of the jaw ("mit den Kieferknorpeln fest 

 verwuchsen "), we become alarmed by his apparent disbelief in 

 some of the accepted fundamental principles of vertebrate anatomy. 

 A mere reference to literature will suffice to correct the extra- 

 ordinary mis-statement contained in the expression, " Placoidei, Ag. 

 =: ElasmobrancJiii, Bonap. = Chondropterygii, Cuv. emend. Gtinth."; 

 and the idea that the calcification of the Elasmobranch endoskeleton 

 always consists merely in a superficial "incrustation," will soon be 

 removed when the author proceeds further in his studies especially 

 of the Palaeozoic genera. 



Legitimate inference from an array of facts is always a welcome 

 incitement towards new research ; but when so many premises are 

 false, theoretical disquisitions are a burden to literature. As long 

 ago recognized by Owen, when he founded the family of Cochlio- 

 dontidas, the peculiar dentition of this group of Sharks results only 

 from the fusion of one or more transverse series of teeth into con- 

 tinuous plates, which proceed to grow at the inner border in normal 

 fashion, and curve downwards outside instead of breaking away. 

 Every stage in this process of specialization is known, from the 

 slightly modified Pleuroplax to the extremely specialized Deltopty- 

 chius ; and the only remains of the trunk of these fishes hitherto 

 discovered (Pleuroplax and DicJielodus) conform to the Cestraciont 

 type — not to that of Menaspis. As Egerton himself observed in 

 the original memoir, the teeth of the Myriacanthidas bear much, 

 superficial resemblance to the dental plates of the Cochliodonts ; 

 and it is thus easy to conceive how they may have been developed 

 at first in a similar manner from a dental armature such as was 

 possessed by the earlier Elasmobranchs. But the teeth even of the 

 Myriacanthidse are already typically Chim^roid in structure and 

 mode of growth ; and to infer from the possession by these fishes of 

 triangular dermal plates, that the Palaeozoic Cochliodonts must be 

 somehow related and armed with the ichthyodorulites named 

 Oracanthus, etc., seems to us an entirely unscientific procedure. 

 We still remain of opinion that the Cochliodonts are a specialized 

 offshoot of the Cestracionts, and that the Menaspidge (as the family 

 typified by Menaspis ought to be termed) represent some unknown 



