MERRIAM: TRIASSIO ICIITITYOSAURIA. 137 



SHASTASAURUS Merriam. 



Merriam, Amer. Jour. Sc. 1895, vol. 4, p. r>li. 

 (Dames). Sit/h. d. A<-ad. d. Wiss. Berlin, 1895, p. 104S. 

 ! ( Bouleii<rer), Zool. Record. Kept. p. 29, 189(i. 

 S<-liaxt(ix(inntx ( Yakowlew), Verh. d. Kais. RHSS. .Min. (ies. Bd. 40. p. 194. 



Vertebral centra relatively short, neural spines often great ly thickened. 

 Hicipital ribs only in anterior part of series. Coracoid pedunculate; scapula 

 expanded distally, and with anterior hook. Pubis and ischiuin much ex])andc(l 

 distally, pubis with deep obturator notch. Propodials and epipodials of an- 

 terior limb relatively much shortened. Epipodial elements separated by a cleft 

 excepting in extreme cases of shortening (S. careyi}. Posterior elements of 

 epipodial and mesopodial series relatively reduced. 



Typical species, S. pacificus, from the upper portion of the Trachyceras 

 Beds of the Hosselkus Limestone of Shasta County, California. 



This genus is characterized by the extreme specialization of the limbs 

 and of the neural vertebral arches. Both propodial and epipodials of the an- 

 terior limb are greatly shortened, the most extreme form appearing in S. 

 ciire/ji, where the length may not amount to more than 75 per cent, of the width. 

 With the reduction in length of these elements there has occurred also a reduc- 

 tion of the posterior elements of the limb, so that the posterior epipodial and 

 probably the third or posterior mesopodial were much smaller than in the other 

 genera. Judging from what is known of this reduction, the posterior digit has 

 also suffered reduction, and the limb may have been reduced nearly to a two- 

 fingered type. The elements of the arches are generally thick and heavy. 

 Particularly is this true in the case of the coracoid, which is also exceptionally 

 narrow anteroposteriorly near the proximal end. 



The skull is fairly well known in this genus, but has not shown any very 

 marked characteristics. 



The species occur in the Hosselkus Limestone of Shasta County, Cali- 

 fornia, ranging from the lower part of the Trachyceras Beds to the massive 

 upper limestones. S. osmonti seems to range from the Trachyceras Beds, 

 where it is common, to the upper, massive limestone where it is rare. S. alex- 

 (itidrae, S. altispinus, and S. pacificus are known from the upper portion of the 

 Trachyceras Beds. In the upper, massive limestones S. careyi is the common 

 form, vertebrae of S. osmonti being seen with it occasionally. S. carijei may 

 possibly occur also in the upper part of the lower horizon. 



The history of the genus seems to show pretty clearly a gradual evolution 

 out of an earlier, smaller, and less specialized type like S. osmonti, to the much 

 larger and more highly specialized S. careyi, which is almost the only form 

 seen in the upper, massive limestones. 



