358 Transactions. — Geology. 



of the section are wanting, , and the u]iper, ov chalk, group is found to rest 

 against the okler sandstone rocks of the district. In the estimated thickness 

 of 2,500 feet of strata exposed at the Amuri Bhiff, it is evident, from the 

 fossils, that the sub-divisions as above may be, at least provisionally, adopted 

 with advantage. The only fossil that appears to be common to the second 

 and third groups is Trigonia sulcata,^ and rinless No. III. is the equivalent 

 of the Belemnite beds at Waikato Heads and Kawhia, it has not yet been 

 found at any other part of the colony than the Amuri Bluff. If it is the same 

 formation the evidence obtained from the above section would seem to require 

 the sub-division of the Putataka beds into two distinct formations. 



Art. LIII. — On tlie Teeth of the Leiodon. By ChXrles Knight, F.R.C.S,, 

 President of the Wellington Philosophical Society. 



Plates XXIY.— XXYI. 



\Re.ad before the Wellington Philosophical Society, 16th January, 1874.] 

 At the siiggestion of our late President, Dr. Hector, I have examined 

 microscopically the fossil teeth of the remains of the Leiodon in the Museum. 

 The aquatic Saurians are arranged under Sauropterygia and Pythono- 

 morpha. The former had two pairs of limbs, the latter an anterior pair 

 only. The Leiodon belongs to the Pythonomorpha, with snake-like bodies 

 of immense length. The L. dyspelor, discovered in New Mexico, is estimated 

 at not less than. 100 feet in length, and would be, says Professor Cope, the 

 longest reptile known, and may well excite our astonishment. 



The Leiodon is closely allied to the celebrated gigantic Mosasaurus hoffmanni, 

 or what was at first called the crocodile of Maestricht. Neither Mantell nor 

 Owen wei'e able to say, from the few and scattered remains to which they had 

 access, whether the Leiodon is a species of Jfosasanr or a distinct genus. The 

 chief distinction is in the teeth, whicli, in the Mosasaur, have the outer side 

 fiat with two sharp edges, while the inner side is round. Where the teeth 

 are absent the unsettled distinction between the Mosasaur and the Leiodon 

 renders it probable that some of the former species may really be J^eiodon, as 

 suspected by Professor Cope in his paper on the fossil rejrtiles of the cretaceous 



* Trigonia sulcata, n. sp. General form like T. glhbosa, but sculpturiug difFcrent. 

 Traversed radially by a wide groove ; posterior area with radial striie ; anterior with 

 divaricate ridges cut into tubercles by conceutric striaj, that are continued over the 

 wliole surface. Valve rather tlat ; hinge-margin rounded and overhaiii^ing ; hinge- 

 plates stroiig. Length, 3' 5 inches ; width, 3 inches. 



