the two main lines 465 



Latipinnatid.1i: and Longipinnatid^ 



In the Triassic the Ichthyosauria already appear, divided into two 

 main lines, the Latipinnatidse and the Longipinnatidae. 



The first known latipinnatid is Mixosaurus, from the Muschelkalk. 

 Its direct descendant in the Lower Lias of England is the genus Euryp- 

 terygius (Jaekel), with the well known species E, communis, interme- 

 dius, and others having ribs with divided heads and pelvis consisting of 

 three separate bones. In the Upper Jurassic the latipinnatid group is 

 represented by several new genera: (1) Macropterygius (= group of 

 I. trig onus, with extremely large paddles, ribs as in OphtJialmosaurus, 

 pelvis consisting of three separate bones, trunk long, tail short, teeth 

 covered at the root with a mantle of cement), which is continued into 

 the Middle and Upper Cretaceous (until Senonian) ; (2) Myopterygius 

 (= campylodon group, in some respects similar to the former, but with 

 enormous trochanters on the humerus and femur, and with differences 

 in the skull) ; (3) the well known Ophthalmosaurus (inch Baptanodon) 

 (intermedium between radius and ulna, vertebral column and ribs sim- 

 ilar to Macropterygius, pubis and ischium coalesced) ; (4) Bi'acliyptery- 

 gius [extremus, only radius and ulna in contact with the humerus, but 

 with large sesamoid ossifications on both borders of the paddle, beginning 

 at its proximal end). 



The longipinnatid line, beginning with Cymhospondylus in the Middle 

 Triassic, is still more highly varied than the latipinnatids, but more so 

 in the early Mesozoic. In some respects Cymhospondylus is still more 

 primitive than Mixosaurus in the proportions of the body, which has (as 

 compared with Ichthyosaurus) an extraordinarily small head and very 

 long and large paddles, especially the posterior pair. All the different 

 shastasaurids have a short life and a rapid development. One line, which 

 can not be included in the shastasaurids, but is related to this group, 

 extends into and through the Lias. For this group I have proposed the 

 generic name Leptopteiygius. Some Ehastic forms, and the well known 

 species /. platyodon, lonchiodon, tenuirostris, etcetera, of the Lower and 

 acutirostris and integer of the LTpper Lias, constitute this genus and are 

 characterized by proximally undivided ribs (but nevertheless with two 

 articulations) and by three separate bones in the pelvis. A special form 

 of this group is EurJiinosaurus (Abel) longirostris in the Upper Lias. 

 So the whole of this highly diversified main line of the Longipinnatidse 

 does not extend beyond the Liassic, and even in the Triassic has dis- 

 appeared almost entirely. 



XXXI — Bull. Geol. Soc. A.m., Vol. 34, 1922 



