Remains of the Pycnodont Fish^ Mesturus. 15 



rays and no gular plate, while the cheek-plates are very 

 irregularly subdivided. It is also curious that the allies of 

 this genus and Dapedius (e. g., Tetragonolepis and Aetheo- 

 lepis) are the only known " Lepidosteoids " in which the 

 caudal squamation degenerates behind a definite straight line, 

 as it does in so many Pycnodonts. On the whole, tlierefore, 

 it is reasonable to refer the fishes under discussion to a place 

 among the " Lepidosteoids " corresponding with that occupied 

 by Acipenser among the " Acipenseroids " and by the 

 Siluridse among ordinary Physostomi. 



EXPLANATION OF THE PLATES. 



Mesturus Leedsi, sp. n.— Oxford Clay, Peterborough. All the figures 

 natural size unless otherwise stated. 



Plate I. 



Fig. 1. Head &c., right lateral aspect, with right mandibular ramus 



removed, one-half nat. size. [Leeds Collection, no. 1.] 

 Fig. 1 a. Ditto, superior aspect, one-half nat. size, art., articular ; d, 



dentary ; eth.., ethmoidal plates ; fr., frontal ; op., operculum ; 



orb., oxiAt; p.eth., posterior ethmoidal; />.o^,, preoperculum ; 



pet., pectoral fin ; pnw., preniaxilla ; ptq., pterygo-quadrate 



arcade ; qu., quadrate ; s.o., suborbitals ; s.occ, supraoccipital ; 



s.t., supratemporals ; sp/., splenial ; sg-. , squamosal ; v., vomer; 



X, parietals ; y, maxilla (?) ; z, undetermined dermal plate. 

 Fig, 1 h. Ditto, vomerine dentition. 

 Fig. 1 e. Ditto, left splenial dentition, wanting outermost series. 



PXATE II. 



Fig. 1. Imperfect head, superior aspect, two-thirds nat. size. [Leeds 



Collection, no. 24.] Letters as in PI. I. 

 Fig. 1 a. Ditto, parasphenoid, inferior aspect, two-thirds nat. size, bjjt., 



basipterygoid process ; k,, knob ; jsas., main part of bone ; pt.f., 



pustfrontal (sphenotic). 

 Fig. 2. Ditto, vomerine dentition. 

 Fig. 3. Ditto, left splenial with dentition. 

 Fig. 4. Ditto, right dentary, outer aspect. 

 Fig. 5. Ditto, right angular, outer aspect. 

 Fig. 6. Ditto, left clavicle, outer and inner (6 a) aspects. 



Plate III. 



Fig.l. Portion of squamation. [Leeds Collection, no. 1.] 



Fig. 2. Mandible. [Leeds Collection, no. 2.] ang., angular ; cor. coro- 

 noid ; d., dentary ; spL, splenial ; t, tesserae. 



Fig. 3. Parasphenoid, inferior and superior (3 a) aspects. [British 

 Museum, no. P. 6834.] bpt., basipterygoid process. 



Fig. 4. Supposed basisphenoid, inferior and superior (4 a) aspects. 

 [Ditto.] a./., anterior facette ; p.f., posterior facette. 



Fig. 5. Eight operculum, outer and inner (5 a) aspects. [Ditto.] /., fa- 

 cette for hyomandibular suspension. 



Fig. 6. Left operculum, imperfect below, outer aspect. [Ditto.] 



