AMIOIDEI 



337 



Family Archaeonemidae. Differing from the last in the possession 

 of thin cycloid scales, and ridge scales along the mid-dorsal and ventral 

 lines. 



Archaeonemus, A. S. W. ; Jurassic, N.S. Wales. 



Family Oligopleuridae. In these the ganoine is vestigial, the scales 

 thin, overlapping, and more or less rounded, the centra annular or 

 amphicoelous, and generally well ossified. The first centrum has two 

 disks. 



Oligopleurus } Thiol. ; Jurassic, Europe. Omoscopvs, Costa ; Jurassic 

 and Cretaceous, Europe. Spathiurus, Davis ; Cretaceous, Mt. Lebanon. 



Fig. 317. 



a. transverse section of the jaws of a Pycnodont, showing the two halves of the splenial 

 dentition opposing the vomerine teeth above ; b, vomerine and splenial teeth of Microdon ; c, 

 vomerine ami splenial teeth of Coelodus ; d, portion of vertebral column of Coclodiis, showing 

 the persistent notochord (shaded) and the expanded bases of the neural and haemal arehes ; e, 

 the same of Pycnodus ; f, inner view of scales, showing- mode of interlocking by pegs and 

 sockets, continued as longitudinal ribs. (Alter J. J. Heckel, from A. S. Woodward.) 



Family Pycnodontidae. This is a highly specialised family of deep- 

 bodied compressed fish found in deposits from the Lower Lias to the 

 Eocene. The dorsal and anal fins are greatly extended backwards, the 

 pectorals small, and the pelvics vestigial (Fig. 319). The caudal fin, 

 externally symmetrical, is of the abbreviate heterocercal type and often 

 strongly forked. Fulcra are absent or vestigial. The scales are rhombic 

 but greatly lengthened transversely (Fig. 317). They usually have a 

 large internal keel which projects above and below, forming articulating 

 pegs, or may fit closely by irregular sutures (Mesturus). The scales may 

 be absent from the tail, and sometimes appear to grow only on the 

 anterior half of the body (Mesodon, Microdon). There is a covering of 

 ganoine. The arrangement of the cranial bones is remarkable and 

 inconstant; it has been variously interpreted (Zittel [512], A. S. Wood- 

 ward [502-3], Henning [208]). The orbits are high up in the head, 



