32 OALIFOnXTA A<AnE.MV ol' SCIENCES 



they liavo most in coinnion are either those eoiiiprisiiig typical atjuatic forms or 

 th(jse in wiiieh rliyncliocepluilian characters are very prominent. Even where 

 we liavc the characters of an a([uatic animal (U'veloped in a form of the rhyncho- 

 eephalian type, we fail in every instance to find closer afhnitv than the l)on(l 

 between the iMosasauria and the Lacci'tilia or tliat between the ( 'horistodera and 

 the Rhynchoceplialia. 



Dnrin^i;' the [leriod in which the Thahittosaurs tloiu-islied there were three 

 other impoi'tant i^ronps of marine rej)tiles in existence. These were tlu' Notho- 

 saurs, Ichthyosaurs and l'laco(h)nts, none of which were closely related to tiie 

 Thalattosauria. Of tiie older reptilian orders, the Protorosauria and Progano- 

 sauria are nearest the Thalattosaurs. The first of these comprises terrestrial types 

 with (jnite a different structure and showing already in the Permian that the 

 tendency of their evolution was toward highly specialized land forms. The 

 Proganosaurs, also of the Permian, exhibited a different type of evcdution in the 

 skull, ril)s and vertebrae, and had much less specialized limbs. 



Of the younger orders the ("horistodera and Iv]iyncliocej)halia show notice- 

 al)le similarity to the Thalattosaurs in their fundamental skeletal plan. The 

 affinity with the ("horistodera is on the whole weak, as the skull, arches and 

 lind)s are decidedly different. They represent a distinct aquatic type which 

 must reach back to the primitive Diaptosauria for a true expression of its affinity 

 with the Thalattosaurs. The Khynchocephalia s. str. as shown in the luimerous 

 types ordinai-ily classed in it show more real kinship with the Thalattosaurs than 

 most of the other orders. Affinities are expressed in varying degrees in Sphcvn- 

 ihni. Uoniensaurnx, PIcnrnsanriis and Sduranndon. It is, however, to be doubted 

 whether all of these forms s!iovd<l be grouped in one order. In the possession of 

 luunerous jieculiar characters, both primitive and specialized, and ])artii-ularly 

 through the group of characters which has arisen owing to the higher degree of 

 adaptation to acjuatic conditions, the Thalattosaurs show themselves distinctly 

 separate(l from the true Khynchocephalia. 



The resend)lances to the S(|uamata are striking. They are evidently fouiide(l 

 in a large measure on parallelism, but may also be due to (dose affinity between 

 the primitive Thalattosaurs and the ])ai'ticular group in the primitive Piaptosaurs 

 from which the S(puimata originateil. 



The Thalattosaurs rejiresent an earlv adaptation to marine c(mditions of 

 that division of the Reptilia which has persisted in measurably priniitiN'e form 

 in the Khynchocephalia. l)uring the early iiistoi'y of that grouj) it ga\(' rise to 

 a numerous company of forms taking (piite divergent paths in their evolution. 

 ( )f the older onlers (uily the Progauosauria were a(iuatic. They appear, how- 

 ever, to have been limite<l to fresh watei'. The Thalattosaurs are eviflentlv the 



