1890.] FROM THE WEALDEN AND PURBECK BEDS. 353 



characters of the teeth is somewhat unfortunate, and this important 

 information must be supplied before the species can be regarded as 

 completely defined. The characters enumerated above, however, 

 are at once sufficient to exclude from comparison all known species 

 except the typical M. macropterus, from the Lithographic Stone ; 

 and, as shown by a nearly complete specimen of the last-named form 

 in the British Museum (no. P. 5546), it is distinguished irom the 

 Purbeck fossil by the maximum depth of the trunk being equal to 

 the combined length of the head and trunk, while the^ head and 

 opercular apparatus together occupy not less than one quarter of the 

 total length of the fish. It is thus evident that we are concerned 

 with a new species ; and, the generic relationships of the fossil 

 having been first recognized by Mr. William Davies (in the Brit. 

 Mus. Register), the name of Mesodon daviesi seems appropriate. 



EXPLANATION OF THE PLATES. 



Plate XXVIII. 



Pig. L Oligopleunts vectensis, sp. nov. ; head, lateral aspect, one half natural 

 size. Wealden ; Isle of Wight. [B.M., no. 42013.] 



ar., articular, d.. dentary. ecpt., eetopterjgoid. enpt., eutoptery- 

 goid. g.r., gill-rakers, km., hyomandibular. mpt., metapterygoid. 

 mx., maxilla, pas., parasphenoid. prtuc., premaxilla. qn., quadrate. 

 S7nx., supramaxillaries. 



1 a. Vertebral centrum, posterior and lateral aspects. 



2. Ditto ; branchial arch and gill-rakers, outer aspect. Middle Purbeck 



beds, Swanage. [B.M., no. P. 4219.] 



3. Ditto ; first vertebral centrum of same specimen, right lateral aspect. 



4. Ditto; vertebrae, lateral aspect. Ihid. [B.M., no. P. 1121.] 



5. Mesodon daviesi, sp. nov. ; lateral aspect of fish, one half natural size 



Ibid. [B.M., no. 41387.] 

 op., operculum. 



Plate XXIX. 



Fig. 1. OUgnplmms vectensis, sp. nov. ; anterior portion of dentary bone, 

 lateral aspect. Jliddle Purbeck beds, Swanage. [B.M., no. P. 4219.] 

 1 a. Tooth, enlarged four times. 



2. Ditto ; preoperculum, two thirds natural size. Hnd. [University Col- 



lege, Bristol.] 



3. Oligopleurus (? immature vectensis) ; lateral aspect offish, nearlv natural 



size. Ibid. [B.M., no. 40423.] 



md.. mandible, mtp., metapterygoid. op., operculum, p.op., pre- 

 operculum. pas., parasphenoid. qu., quadrate. 



4. Strobilodus ptirbeckensis, sp. nov. ; head, &c., lateral aspect. Ihid 



[B.M., no. 46911.] ^ 



cL, clavicle, mx., maxilla, pmx., premaxilla. p., pectoral fin. 

 v., vertebrse. Other letters as before. 



B.M.=British Museum. Unless otherwise stated the figs, are of the natural 

 size. 



