Geological Society. 351 



and perhaps the finest Pliosaur skull known. It bears a great 

 similarity to Peloneustes philarchus, but there are a number of 

 differences which tend to show that the subject of the present 

 communication is not the skull of an old individual of Peloneustes. 

 Although the teeth of the fossil here described agree precisely with 

 those described by Sauvage from the same horizon at Boulogne 

 under the name Liopleurodon ferox, they differ considerably from 

 those of the Kimeridge Clay upon which Owen founded the genus 

 Pliosaurus ; they, however, show a distinct tendency towards the 

 typical form, and since the skull and skeleton of the Oxfordian and 

 Kimeridgian forms are, so far as known, closely similar, the Author 

 prefers for the present to follow the British Museum Catalogue in 

 referring them both to one genus, Pliosaurus. 



The Author gives a detailed description of the skull which forms 

 the subject of the paper. 



February 3, 1897. Dr. Henry Hicks, P.R.S., 

 President, in the Chair. 



The following communication was read : 



1. ' The Subgenera Petalograptus and GepTialograptusJ By 

 Miss G. L. Elles. 



The forms referred to in the paper are accepted as subgenera 

 of Diplograptus, as defined by Lapworth. The two subgenera 

 have frequently been much confused, but examination of specimens 

 preserved in relief shows that they have very distinctive characters, 

 especially at the proximal ends. The Author gives diagnoses of 

 the two subgenera, and detailed descriptions of the following 

 forms : Petalograptus folium, His. ; P. palmeus, Barr., and varieties 

 latus, Barr., tenuis, Barr., ovato-elongatus, Kurck ; P. ovalus, Barr. ; 

 P. n. sp. ; Ceplialograptus cometa, Gein, ; and G. n. sp. 



She concludes that Petalograptus has been derived from Ortho- 

 graptus foliaceus, 0. truncatus having been a step on the way. The 

 latter form has an almost horizontal connecting-canal, so that the first 

 of the second series of hydrotheca3 arises at nearly the same level as 

 the first of the primordial series ; whilst, if the connecting-canal 

 became more oblique and the theca3 more concavely curved, a form 

 identical with P. palmeus would be the result. Purther changes 

 would give rise to P. n. sp., and subsequently to P. folium. When 

 the first theca of the second series arises so late that the sicula is 

 entirely free on the side remote from that on which the first of the 

 primordial series arises, an important stage is reached, and the form 

 becomes a Cephalograptus. Such a form is furnished by G. n. sp., 

 which is in some respects intermediate between Petalograptus and 

 Geplialograptus. The extreme form is reached in G. cometa, in 

 which the first hydrotheca of the second series is still later, the 

 hydrothecse are still longer than those of earlier forms and almost 

 parallel to the long axis of the rhabdosoma. The other known 

 forms of Petalograptus may have been derived from P. palmeus. 



