264 MR. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [vol. Ixxvii, 



Liassie fauna are best displayed near the summit of the pass 

 between Oroya and Tarma, where they consist of j^ellow, impure, 

 sandy limestones, compact grey limestones Avith abundant chert- 

 concretions, and glauconitic marls. 



They here form one limb of a sharp overf old directed westwards ; 

 the dip changing from one of 80° south-w^estwards at the top of 

 the cliff- face to 50° north-eastwards at the base. Fossils are 

 abundant, and for the greater part are silicified and well-preserved. 

 The most important determinative forms are comprised in the fol- 

 lowing list : — JEchioceras cf. raricosfatum (D'Orbigny), Arietites 

 aif. conyheari Sowerby, Terebrcdida (Lohothyris) cf. ovatissima 

 Quenstedt, and Vola cdaia Yon Buch. 



Palseontological Note. 



EcHiocERAS c/. RARicosTATUM (D'Orbigny). 



Specimens of this ammonite, though abundant, are somewhat poorly- 

 preserved, but their widely-umbilicate, planulate form, coupled with the 

 simple raricostate type of ribbing, leaves little doubt as to their correct 

 determination, and they thus form a useful index to the age of the beds. 



Teeebratula (Lobothyris) c/. ovatissima Quenstedt. (PI. XV, figs. 1-6.) 



The general form of the shell, its various growth- stages, and the nature of 

 the muscular impressions, suggest a typical Liassie species of the genus 

 Lobothyris. The majority of specimens appear to be closely related to 

 T. ovatissima of Quenstedt from the Sinemurian and Hettangian of Germany, 

 which, as pointed out to me by Mr. S. S. Buckman, being a more advanced 

 form than the later Domerian species T. jjujicfata, must be regarded as 

 belonging to a branch- stock and not as the ancestor of the latter. 



The marginal contours of the shells that are grouped together here show 

 considerable variation in different individuals. There is, however, no justi- 

 fication on this account for drawing specific distinctions between the various 

 forms, for they appear to belong to a definite morphogenetic series, and 

 difPer merely in the stage of development attained when growth was arrested. 

 The two main types of shell-outline (a and /3) illustrated by F. A. von 

 Quenstedt,^ which for convenience may be termed the ' obese ' and ' elongate,' 

 are both represented in my collection, and their respective growth-stages are 

 found to be parallel. The ontogenetic development, as represented by the 

 single character of marginal contour, exhibits the following sequence of 

 stages, which conform with the law for Terebratulid morphogeny propounded 

 by Mr. S. S. Buckman - :— 



Biconvex, Eectimarginate stage. 

 Uniplicate (Lophrothyrid) stage. 

 Sulciplicate (Terehratida) stage. 



' Rectimarginate ' forms are usually of small size (PI. XA^, fig. 3) and the 

 majority of larger specimens have attained the uniplicate stage (PI. XV, 

 figs. 1 a & 1 h, 4). The more advanced or ' sulciplicate ' stage is less common, 

 but examples of both types of shell showing this character were found in a 

 glauconitic marl at a somewhat higher horizon (PI. XV, figs. 5 & 6). 



Elongation of the shell, resulting in the second or ' elongate ' variety 



^ ' Petrefactenkunde Deutschlands ' vol. ii, Brachiopoden (1871) p. 328 & 

 pi. xlvi, figs. 54-55. 



2 ' Brachiopoda of the ISTamyau Beds, Northern Shan States (Burma) ' Mem. 

 Geol. Surv. India : Pal. Indica, n. s. vol. iii, Mem. No. 2 (1917) p. 83. 



