\ 

 340 AMIOWEI 



The notochordal axis shows no trace of centra, and was probably per- 

 sistent and unconstricted. The neural and haemal arches are well ossified 

 and continuous with the spines, become greatly expanded, and develop 

 articulating processes (Fig. 317). The pleural ribs expand in the same 

 way. The bases of these arches tend to grow round the notochord to a 

 small extent in Coelodus, to a great extent in Pycnodus, where the dorsal 

 almost meet the ventral. 



Many of the most important structural characters of the Pycnodontidae 

 are correlated with the development of a grinding dentition. Although 

 their systematic position cannot be considered as definitely established, 

 yet their removal from the Platysomatidae by Traquair and A. S. Wood- 

 ward has brought to light their affinity to the Semionotidae. In the 

 structure of the vertebral arches they approach the Teleostei. 



Mesodon, Wagner (Fig. 319); Mesturus, Wagner; Microdon, Ag. ; 

 Jiirassic, Europe. Athrodon, Sauvage ; Gyrodus, Ag. ; Coelodus, Heckel ; 

 Jurassic and Cretaceous, Europe. Coccodus, Pictet ; Xenopholis, Davis ; 

 Cretaceous, Mt. Lebanon. Pycnodus, Ag. ; Cretaceous and Eocene, Europe. 

 Palaeobalistum, Blainv. ; Cretaceous and Eocene, Europe and South 

 America. 



Order 2. LEPIDOSTEOIDEI. 



This order dates only from Eocene times, and contains a single 

 genus, Lepidosteus (J. Miiller [307], Balfour and Parker [30 and 320], 

 van Wijhe [494], Kolliker [271], Collinge [85], Allis [19]). The 

 body is elongate, the dorsal and anal fins short, and the caudal 

 abbreviate-heterocercal (Figs. 324, 62). Thick rhombic ganoid 

 scales cover the trunk and tail. These scales may bear numerous 

 small true denticles on their surface (Fig. 192), and are provided 

 with peg and socket articulations. The dermal bones have a similar 

 layer of ganoine. A long snout is produced by the elongation of 

 the jaws in a manner which is quite characteristic in that the 

 nostrils and nasal sacs are carried to the extremity. It is therefore 

 the ethmoid region which is lengthened, and the olfactory nerves 

 reach the nasal capsules through long canals in the ethmoid cartilage. 

 Paired fulcra arm the edges of the fins (Fig. 62). On the skull 

 are to be noticed the usual paired supratemporals, parietals, and 

 f rontals ; the pterotics are large, and the nasals are represented by 

 elongated ethmo-nasals and small anterior nasals. There is no 

 pineal foramen. A long preoperculum runs along the ventral edge 

 of the operculum, most of the cheek region being covered by 

 numerous irregular plates (Fig. 320). Characteristic of the upper 

 jaw is the subdivision of the maxilla into a row of several toothed 

 bones. The teeth are strong and pointed, with the wall radially 

 folded at the base (Fig. 81, B). There is an extensive interorbital 

 septum, the large exoccipitals meet above the foramen magnum, 

 and no myodome is developed (Fig. 321). An articulation takes 

 place between the prootic and the meta-pterygoid. The hyoquadrate 



