334 Dr. C. Liitken on the Limits and 



fl. The Neozoic (Eocene*) Pycnodonts had the semivcrtebrte 

 developed, and consequently covering the notochord entirely ; 

 the dermal ribs, Avhich are sometimes delicate and compli- 

 cated, never occupied more than the thoracic portion of the body. 



III. Third series. — Tlie Gano'idei Crossopteri or Polypte- 

 ridce^ represented in the present day by the genera Polypterus 

 and Calamoicldliys. The principal characters common to 

 these and tlieir ancient representatives of the Devonian sys- 

 tem are the following : — 1, the absence of rays in the branchio- 

 stegal membrane, which are represented here only by two 

 gular plates ; 2, the very characteristic structure of the paired 

 fins, which are formed of a scaly stem, often of great length, 

 and bordered on each side with rays like a fringe ; 3, tlie very 

 backward position of the ventral fins ; 4, the absence of the 

 so-called fulcral scales ; 5, the dipliyocercal or approximately 

 heteroccrcal form of the tail, which is never fan-like. 



The true Polypteridoi of the existing period are the direct 

 representatives of the Pala30zoIci('/^w»7W/^>/er« (Devonian and 

 Carboniferous) with ossified, rhomboidal, and articulated scales 

 like those of the Lejridostei and Polyptcri^ with a dipliyocercal 

 or slightly heteroccrcal tail, with a double dorsal fin thrown 

 fiir back, with the base of the vertical fins scaly, &c. The 

 principal character which separates them from the Polypteridai 

 therefore consists in the double dorsal placed far back. These 

 are the genera Osteolepis^ Dijilopterus^ Mefjalichthys (witli 

 smooth scales), Glyptolcemus and Glyptopomus (with the scales 

 and bones of the head sculptured). 



The contemporaneous Gydodipteridce present exactly the 

 same assemblage of characters, with one single exception — 

 that of the scales : these are ossified and enamelled, indeed, 

 and sometimes even thick and smooth or sculptured, as in the 

 preceding ; but in place of the form, relative position, and 

 articulation common to the Eugano'idei, Rhomhodipteridcey and 

 PoJypterida'^ Ave find here the rounded cycloid form and the 

 imbricated superposition of the ordinary Teleostei. As among 

 the R]ioml)odipte7'id(e, there are among the CyclodipteridcB a 

 smooth division [Ctenodus^ Dipterus), and another with the 

 cranium and scales sculptured {Glyj^tolepis, IIoIoj)tychius, 

 Gyro]}tycliius^ &c.). 



In a certain number, at least, of these DipteridcK^ whether 

 rhomboidal or cycloid, if not in all, the vertebral column 

 already possessed apparently a degree of development little, if 

 at all, inferior to that of the Polypteri of the present day ; in 



* A single species of this tribe is obtained from the Cretaceous forma- 

 tion of Lebanon. For further details concerning the true Pycnodonts, 

 their structure and classification, the reader will consult especially the 

 celebrated works of the late M. Heckel. 



