Ko. 136.] 67 



York a thin layer of limestone in the shale, which is almost 

 made up of encrinal columns, often of large size ; and is known 

 as the Encrinal limestone. It does not extend far to the east, 

 however.) 



The Corals of the Hamilton group are chiefly of forms allied 

 to the columnar and horn-shaped Corals of the Upper Helderberg 

 limestones. The figure given (Fig. 31) explains their more com- 

 mon character. There are, however, many beautiful small forms 

 belonging to the Bryozoa, which are found adhering to the inner 

 or outer surfaces of shells on which they seem to have been para- 

 sitic. 



The bivalve shells of this rock are exceedingly numerous. 

 There are many species of Avicula (Fig. 32, No. 3) ; and many 

 of oblong forms bearing some general resemblance to the " fresh- 

 water muscles " of our lakes and streams, though all are of 

 marine origin (Fig. 33). There are great numbers of brachiopo- 

 dous shells, of which some of the most marked are the great 

 DisciNA (Fig. 32, No. 4), and the Atrypa concentrica (Fig. 34, 

 No. 5), an exceedingly common species. The Jltrypa reticularis 

 (Fig. 34, No. 4) is another common form, which is known through 

 a great thickness of strata; being found in rocks far below the 

 Hamilton group, even as low as the Clinton, (if indeed the species 

 be identical throughout.) A beautiful bivalve shell is the Gram- 

 MYsiA, which has its knobs or umbones curved into two coiled 

 spires which approach each other at the hinge, with a broad fur- 

 row running from each umbo diagonally across each valve to the 

 margin at its other end. The Nucule (Fig. 33, Nos. 4, 5) are 

 small, short and thick bivalves, with the hinges marked with a 

 long line of teeth almost like a saw, seen (like all such details of 

 structure) best in broken or weatherworn specimens. Among the 

 univalve shells are species of Bellerophon (Fig. 34, No. 1), ex- 

 plained under the head of the Trenton limestone ; Pleuroto- 

 MARiA, small and often beautifully marked snail-like shells with a 

 band on the outer edge ; Loxonema (Fig. 30, No. 8), a long spiral 

 form with lines crossing each whorl or coil ; and Natica, a coiled 

 shell formed much like a large snail, but wanting the marginal 

 band found in Pleurotomaria. We may add some of the Spiri- 

 FERs, very long or rather wide and pointed braehiopodous shells; 

 of which several are shown in the woodcuts (Fig. 29, No. 3 ; 

 Fig. 34, No& 2, 3 ; Fig. 30, No. 5). 



