278 



FOSSILS. 



Fio. 177. 



Maclurea magna. 



seem to be entirely composed of the shells. It is very near the top of the Chazy limestone. At the time of 

 our visit there we thought we detected certain fossils of the Trenton limestone in connection with the 

 Rhynchonella allilis. 



Rhynclionella plena (Hall.) Our notes state 

 that Ehynconella plena is also found at Merriam's 

 Bay. This shell is "somewhat quadrangularly gib- 

 bous or rotund ; front margin elevated in a moder- 

 ately deep sinus ; beak of the dorsal valve small, 

 closely incurved over the beak of the ventral valve ; 

 surface marked by sixteen to twenty strong radii, 

 about four or five feet of which are depressed, 

 forming the sinus of the dorsal valve, and an 

 equal number elevated above the others on the 

 ventral valve." 



Maclurea magna (Hall.) This is a large beauti- 

 ful univalve gasteropod mollusk, which is the best 

 characterized species of the Chazy limestone. 

 The description of the genus has already been 



given under calciferous sandrock. The species 

 M. magna is described in the following language : 

 " Sinistrorsal, discoidal, depressed turbinate ; 

 breadth more than twice as great as the height ; 

 spire flat, a slightly depressed line at the sutures ; 

 whorls about six, gradually increasing from the apex, ventricose, flattened above, obtusely angular on 

 the outer edge ; surface marked by fine striae, which, upon close examination, are found to be produced by 

 the imbricating edges of lamellae ; striae undulating, bending backwards from the sutures, and forwards in 

 passing over the edge of the shell ; aperture obtusely trigonal, depressed above, slightly expanded beyond 

 the dimensions of the whorl just behind it ; axis hollow ; umbilicus broad and deep, extending to the top 

 of the spire." 



Fig. 177 represents the upper surface of the Maclurea magna. This is only one of many positions in 

 which it may be seen upon the surface of ledges. Sometimes the shell has been distorted by pressure ; as 

 is the case in several specimens in the State Cabinet from the northwest part of Ferrisburgh. 



Wherever the Chazy limestone occurs in Vermont, we may expect to find the Maclurea magna. It is 

 abundant at Larrabee's Point in Shoreham; at Judd's quarry of black marble; in a great many places in 

 Bridport; in Addison, particularly at the base of Snake Mountain; in the west part of. Pan ton, where some 

 of our best specimens were obtained; in Vergennes; on the west side of Bush Mountain, southeast of Ver- 

 gennes; in the west part of Ferrisburgh; in Charlotte; all along the west part of South Hero; and in the 

 southern portions of Isle La Motte, particularly in the limestone at Hill's south quarry. One of the best 

 places to procure specimens of it for cabinets, is at the mill at Swanton Falls, where this rock is sawn for 

 architectural purposes. There are many buildings in Vermont made of the Chazy limestone, in which this 

 beautiful shell may be seen to advantage. 



Because of the wide geographical distribution and the limited geological range of this shell, it is of great 

 value in the identification of strata. Prof. James Hall says that "in the contorted and partially altered 

 limestones of Eastern New York and Vermont, where nearly all other remains are obliterated, I have found 

 the various sections of this shell, often compressed or distorted, but frequently, and almost always sufficiently 

 clear to identify the mass." 



Cyrtoceras. In Addison, at the base of Snake Mountain there occurs an undetermined species of 

 Cyrtoceras, in connection with Maclurea magna. The specimens of it are quite numerous. The charac 

 ters of the genus are these : Shell curved or partially involute, sometimes with the transverse, at others the 

 longitudinal diameter the greater. Aperture often contracted (in the smooth forms.) Siphuncle subin- 



