Cretaceous Fish Plethodus. 361 



some of the existing Osteoglossidse, a family which was 

 already differentiated in the Eocene in North America 

 [Dapedoglossus) and probably also in Europe (Bnjchcetus). 

 If the patches of minute teeth on the parasphenoid and basi- 

 hyal of the recent Arapaima were fused together into opposing 

 plates their structure would not be very different from that of 

 Plethodus. The disposition of the squamosal and parietal 

 bones in the Cretaceous fish limits comparisons to primitive 

 bony fishes such as the Osteoglossida?, Elopidse, and AlbulidaB. 

 On the whole, I am inclined to think that Plethodus will 

 eventually prove to be most closely related to the first of 

 these families. 



EXPLANATION OF THE PLATES. 



Plate XIII. 



Fig. 1. Plethodus expansus, Dixon; lower dental plate, oral face, two 



thirds nat. size, and view of tuberculated rim (1 a), nat. size. — 



Cambridge Greensand. [B.M. no. 35369.] 

 Fig. 2. Ditto; upper dental plate, oral face, aboral face (2a), and side 



view of supposed posterior end (2 b), two thirds nat. size. — 



L. Chalk ; Kent, p, median bony process ; x, lateral bony 



plate. [B.M. no. 38585.] 

 Fig. 3. Ditto ; upper dental plate, aboral face, two thirds nat, size.— 



Ibid. [B.M. no. 39091.] 

 Fir/. 4. Ditto (?) ; lower dental plate, oral face, two thirds nat. size. — 



Gault; Folkestone. [B.M. no. P. 7.] 

 Fiq. 5. Plethodus pentagon, sp. n. ; lower dental plate, oral face. — 



L. Chalk; Burham. [B.M. no. 41716 «.] 

 Fig. 6, Ditto ; pointed end of plate, oral face.— Ibid. [B.M. no. 47947.] 

 Fig, 7, Ditto (? ) ; upper dental plate, oral face, and lateral aspect (7 a) . 



—English Chalk. [B.M. no. P. 5626.] 



Plate XIV. 



Fig.l. Plethodus oblongus, Dixon (?) ; imperfect head, right and left 

 (la) lateral aspects.— L. Chalk; Clayton. [Willett Coll., 

 Brighton Museum, no. 154.] 



Fiq. 2. Ditto ; imperfect head, left lateral aspect. — L. Chalk ; Dorking. 

 [B.M. no. 49895.] 



d., dentary of mandible ; ecpt, ectopterygoid ; fr., frontal ; h, lower dental 

 plate (? basihyal) ; hm., hyomandibular ; m., mucus-pit ; m.eth., 

 mesethmoid; m.pt., metapterygoid ; mx., maxilla; op., oper- 

 culum; p.op., preoperculum ; pa., parietal; pr.fr., prefrontal 

 plate ; pt.fr., postfrontal plate ; qu., quadrate ; r, rostrum ; s.t, 

 supratemporals (?) ; s(j., squamosal; t, inner mandibular teeth ; 

 u, upper dental plate ; x, supraoccipital (?). 



B.M. = British Museum. Unless otherwise stated, the figures are 

 of the natural size. 



