CLARK: THENTON LIMESTONK AT MAKI INSIUHG. 17 



It will be noted that all the specimens show plications on the sides 

 of the fold and sinus. Moreover, there are obviously two distinct 

 groups, one with four plications in the sinus of the young, and the 

 other with only three plications in the sinus of the adult. The first 

 five belong to the first group, and also, probably, the last one, while the 

 other four belong to the second. The two specimens at the head of 

 the list are evidently immature, as is shown by their thinness and their 

 very short plications. ^Yith further growth new plications are added 

 in the fold and sinus as well as at the sides, and number five is probably 

 a typical adult. 



No very young specimen of the second group has been found, but 

 the presence of only three plications in the adult shows that it could 

 not have been developed from such young as numbers one and two. 



Only one specimen was collected from the Upper Trenton, a pedicle 

 valve found about 400 feet above the base. It agrees with the second 

 type in the table above, being 10 mm. long, 12 mm. wide, with three 

 plications in the sinus and two on the sides. 



Miss Wilson, in her studies of Parastrophia hemiplicata from about 

 Ottawa (Mus. bull. 2, Geol. surv. Canada, 1914) found specimens 

 with three to five plications in the sinus and plications on the sides of 

 the fold in both Middle and Upper Trenton. 



Gastropoda. 



SiXUITES CAXCELLATUS (Hall). 



BeUerophon bilobatiis Emmons, Geol. X. Y., 1842, 2, p. 392, fig. 6. 



This species was found to be exceedingly common at horizon 70. 

 It also occurred at horizons 15, 20, 390, and 410, but was not seen in 

 the Middle Trenton. In the basal beds, where some other forms 

 were so common, it was absent. At horizon 70, in actual numbers, 

 as the following list shows, it many times exceeded in abundance all 

 other forms collected. The fossils from this horizon, with the number 

 of specimens collected, which the writer can attest to be a fair index 

 to their relative abundance, are listed below: — 



9 Prasopora simulatrix v. occidentalis Ulrich. 1 ScMzocrania filosa 

 Hall. 106 Sinuites cancellaius (Hall). 6 S. cancellatus v. corrugalus 

 (Hall). 1 Pleurotomaria (Trochoncma f) amhigua Hall. 1 Hormotoma 

 trentonensis (Ulrich &Scofield). 4 Ctenodonta levaia (Hall). 3 Caly- 

 mene senaria Conrad — a total of 131. 



