24 CALIFoItMA ACAOKMY (IF SCIE.NCKS 



in which it could he phiccd would be comprised mainly of acjvuitic forms. Its 

 relationship to any other than an a(|uatic jjroup will probably be a weaker bond 

 tlian ordinal affinity, and very close sinnlarity must be souiiht in sdine of the 

 natatory types. 



The characters of the Thalattosaurs are such as to brinji' them into more or 

 less definite relationship to a consideral)le number of ijroups. In some cases 

 this is due to similar adaptation. To several (juite different groujis they show 

 true affinities of nearly the same degree, indicating relationship through a com- 

 mon ancestral type or group from which they have all originated. The most 

 important resemblances and affinities are discussed somewhat in detail before 

 presenting th-e conclusions reached through these comparisons.- 



Iclitlnjiixdiiriini Affiiiitie.^i. — The Thalattosaurs inhabited the same seas with a 

 large group of Ichthyosaurians and showed much the same Icind of adaptation 

 carried also to a high degree. The Ichthyosaurs are generally helil to be nearly 

 relate<l to the Rhynchocephalia, and the Thalattosaurs are evidently near rela- 

 tives of that group. Under these circumstances we should expect to lind the 

 Ichthvosaurs and Thalattosaurs showing similar structure and giving evi<lence 

 of near relationsliip. 



In the extremities, where adajitation to ac^uatic conditions is most readily 

 and distinctly expressed, we find both forms exhibiting the typical natatory 

 type. In the Ichthyosaurs the [iropodial and epipodial elements show a some- 

 what greater degree of abbreviation. Unfortunately we do not as yet know the 

 character of the termimil portions of the paddles in the Thalattosaurs and can 

 nmke no comparison here. As far as is known, the iclithyosaurian paddles seem 

 to have attained a slightly higher degree of specialization. 



In the vertebrae of the tAvo groups there are considerable differences in the 

 character of the neural spines, in the zygapophyses and in the attachment of the 

 up})er arches to the centra. The difference in form is particularly noticeable in 

 the anterior caudal region. The pectoral girdles are somewhat similar, but as 

 far as is known the pelvic arches are c^uite different. 



In the skull we find the nares occupying much the same position in both 

 groups, and the premaxillaries are elongated in both. In the Ichthyosaurs the 

 premaxillary elongation is extreme and the nnixillaries are not only relativcdy 

 small but seem actually to have suffered recluction. Distinctive skull characters 

 separating the Ichthyosaurs widely frcnu the Thalattosaurs are noted in the 

 absence of an inferior temporal opening and of a coronoid elevation, the separa- 

 tion of the postfrontal and postorbital, the enormous size of the nasals and their 

 different relations to the premaxillaries, the different relations of the parietals 

 to the small frontals and to the superior temporal openings, a different structure 



