Classification of the Ganoids. 333 



especially characterized by peculiar dermal ribs * wliicji 

 protected their sides, at least on the anterior part of the body, 

 and which Jield suspended the scales, which are sometimes 

 very delicate, and are rhomboidal, and not articulated, but 

 interlocking- in a very peculiar manner. Generally there 

 is also something very characteristic in the form of the 

 body, which enables us at once to distinguish this well- 

 marked and very remarkable extinct type. If we knew only 

 its most recent representatives, we might doubt as to their 

 true position in the system, so widely do they depart from the 

 Euganoid t^^i^e ; but there is an uninteiTupted series, leading 

 directly from the Eocene Pyenodonts to the Palaeozoic Platy- 

 somi, which no one has ever thought of excluding from the 

 Ganoidei, and showing evidently the filiation of all these 

 creatures. It is a peculiar branch which separated during the 

 Carboniferous period from the common trunk of the Ganoids, 

 and continued in the course of time to depart more and more 

 from its starting-point, to become developed in a more and 

 more perfect manner, and to spread out into a multitude of 

 well-marked genera, until it reached the term of its existence 

 during the Eocene period. The classification of the Lepido- 

 "pleuridoi will reproduce before us the image of this zoological 

 progress : — 



a. The Palaeozoic Lepidoj^huridce or Platysomiiy with the 

 scaling of the body and the dermal ribs completely deve- 

 loped, with fulcral scales bordering the fins, with a naked 

 notocliord, and semivertebrae but slightly or not at all 

 developed, &c. Platysomus and allied genera belong to the 

 Carboniferous and Permian formations. 



b. The Liassic Pleurohpidldai differ from the Stylodont 

 Platysomii only by their well-marked homocerceity. 



c. The true Pycnodontes of the Jurassic, Cretaceous, and 

 Tertiary periods are also homocercal, but the fulcral scales 

 are wanting ; the semivertebrte are more or less perfectly 

 developed. Their very characteristic and diversified dentition 

 furnishes excellent generic characters. 



«. The Mesozoic Pyenodonts had the notochord partly naked, 

 the development of the semi vertebra; being less perfect. The 

 dermal ribs in some formed a trelliswork all over the body 

 as in the preceding, in the others only on the anterior part, as 

 in the following. 



* I have here followed the opinions of M. Heckel with regard to this 

 part of their organization. According to Sir P. Egerton, these dermal 

 ribs are only the anterior and thickened portion of the scales. In tlie 

 question of classification, with which we are here occupied, this differ- 

 ence is of little importance ; the character is persistent, even if the mode 

 in which it has been expressed should prove to be false. 



Ann. &Marj. N. Hist. Scr. 4. Vol. vil. 24 



