part 3] THE0I7GH the andes of peeu akd Bolivia. 265 



commences in the ' rectimarginate ' stage (PI. XV, fig. 2). A further distinct 

 sequence of changes is also illustrated by several specimens in my collection. 

 In these the ' rectimarginate ' stage is followed by a ' sulcate ' or concavi- 

 convex stage, in which the first sulcus is developed on the dorsal valve, the 

 change taking place at an early period of growth. This ' sulcate ' stage 

 persists in the majority of adult forms, but in one example from the higher 

 glauconitic horizon there is a suggestion of a transition to a ' paraplicate ' 

 (Holcothyrid) stage, incipient folds being developed at the sides of the dorsal 

 sulcus. 



In all the above-mentioned forms the beaks are typically Epithyrid, and in 

 the elongate variety of shell there is a tendency towards the extension of the 

 anterior (dorsal) margin of the rim over the dorsal umbo, a condition which 

 Mr. Buckman has termed ' foramen labiate.' 



The Dallininas group is represented by a single example. This has a 

 Mesothyrid beak with well-marked beak-ridges, and is provided with a strong 

 median internal septum in the dorsal valve. 



Owing to the silicified nature of the specimens, attempts to develop the 

 muscular impressions by burning, the method advocated by Mr. Buckman, 

 were not successful ; but it was found that, by prolonged immersion in dilute 

 acid, much of the interior filling could be dissolved and the shell thereby 

 rendered semi-transparent, when the muscle-scars became visible. 



These in forms of small size are narrow and sub-parallel ; but in larger 

 examples they appear to be somewhat divergent, and are swollen at the 

 anterior end. A faint median septum is also visible. 



VOLA ALATA VoU Buch. 



This is a very abundant and characteristic species, and, according to 

 W. Moricke,^ is very common in the Middle and Upper Lias, though rare in 

 the Lower. For figures and description of typical specimens, see C. E. 

 Bayle & H. Coquand, ' Memoire sur les Fossiles Secondaires recueillis dans 

 le Chili par M. Ignace Domeyko ' Mom. Soc. Geol. France, ser. 2, vol. iv 

 (1851) p. 14 & pi. V, figs. 1-2. 



ALBIAN. 



A reference to that part of the section Avhich Hes between Yauli 

 and Oroya shows that the beds are here bent into a great asym- 

 metrical synclinal fold, of which the eastern limb is the more 

 steeply inclined. The gently-dipping beds of the western Hmb 

 are readily accessible in a cutting on the railway at Saco, where 

 they consist of dark bituminous limestones containing numerous 

 fossils of Albian age. 



The following list was recorded from this locality : — 



Diploceras cormitum (Pictet). ; Nncitla jyernana Gabb. 



Brancoceras aff. varicos^im (Sowerby). \ Cyrena huehneri Steinmann. 



Schloenhachia multijida Steinmann. • Tellina peruana Gabb. 

 Schl. cf. acutocarinata (Shumard) Protocardium appressxim Gabb. 



Bose. Pinna cf. rohinaldina D'Orbigny. 



ScTd. cf. helhiapi (Marcou) Bose. Astarte sp. 



Schl. cf. ventanillensis (Gabb). Trigonla sp. 



Acanthoceras cf. lijelli (Leymerie). ^ Tylostoma cf. chihnaliuense Bose. 



Similar beds containing many of the forms enumerated above 

 are also met with between Kumichaca and Yauli, and again at 

 Pachachaca. 



» Bibliography, § IV, No. 43, p. 41. 



