C. B. Keyes — Sedentary habits of Platyceras. 269 



Explanation of figures in Plate VI. 



Fig. 1. Typical, unaltered mica schist. River bank near Cruger's Station ("Z" on 

 the map), No. 21. Section cut approximately parallel to the foliation, shoe's 

 the biotite mostly in basal sections of varying thickness. Large flakes of mus- 

 covite irregularly scattered through the field. "Occasional crystals of tourma- 

 line and zircon. Groundmass mostly a granular aggregate of quartz with very 

 little feldspar. ( x 30). 



Pig. 2. Fibrolitic schist, collected a short distance north of the last on the line of 

 Section I (M. of Professor Dana's collection). Fibrolite very abundant in large 

 sheaf-like bundles, and also in radiating tufts, garnet and biotite. Groundmass 

 composed of coarse grains of quartz with some feldspar. ( x 30). 



Pig. 3. Staurolitic mica schist ("o " on the map) No. 25. Large crystals of yellow 

 staurolite with irregular boundaries and the usual quartz inclusions at their 

 center scattered through a matrix of quartz and biotite. Some fibrolite in 

 radiating tufts. ( x 30). 



Pig. 4. Fibrolitic schist from very near the contact ("p " on the map) No. 37. 

 Rock very largely composed of fibrolite in feathery tufts, bundles and sheaves. 

 Matrix almost wholly a brown mica. There also occur in lesser quantity 

 cyanite, garnet and staurolite. ( x 30). 



Pig. 5. Black inclusion in the mica-diorite (near "s" on the map) No. 29. Com- 

 posed mainly of pleonaste in small octahedral crystals among which are scat- 

 tered larger crystals of colorless corundum. Where the section is sufficiently 

 transparent, feldspar and biotite in small quantity may be recognized. ( x 30). 



Pig. 6. Inclusion in mica-diorite ("s" on the map) No. 12. The most important 

 constituent is margarite in colorless crystals. Other minerals are green mica 

 (ripidolite), biotite, quartz, feldspar and magnetite. Tourmaline and epidote 

 also occur in these inclusions, although they are not represented in the figure. 

 (x30). 



Art. XXYII. — The Sedentary Habits of Platyceras / by 

 (3. K. Keyes. 



The genus Platyceras was founded by Conrad* in 1840 for 

 a Paleozoic group of gasteropodous shells " sub-oval or sub- 

 globose, with a small spire, the whorls of which are sometimes 

 free and sometimes contiguous : the mouth generally campanu- 

 lated or expanded." Hitherto these fossil shells had been 

 referred to the genus of modern Mollusca Capuhis, proposed 

 by Monfortf in 1810. Conrad's name, however, for this fossil 

 group was not until within the past few years generally 

 accepted, preference having been given by most European 

 writers and also by some American authors to Acrocidia of 

 Phil lips, £ notwithstanding the fact that the type of Phillips's 

 genus was a typical form of Platyceras. Although the forms 

 of this genus present so few classificatory characters of definite 

 specific value more than three hundred species have been 

 described — over one-half of which are from America. In this 

 genus, as in many other Paleozoic- genera, numerous species 

 have been based not on any apparent distinctive character, but 

 seemingly simply on their occurrence at different geological 



* Ann. Rep. Palse. N. T., p. 205. f Conch. Syst., vol. ii. 



X Palse. Foss. Cornwall, etc., 1841. 



