32 ■ PALEONTOLOGY OF NEW YORK. 



With the increase of the Aperturati, the OstioLati diminish rapidly and in 

 the upper Devonian fiiunas we know but a single species, S. asper, from the 

 Chemung group of New York.* 



VI. Glabrati. Typical examples, Spirifer glaber, Martin, Martiniopsis inflata, 

 Waagen. 



Forms with the surface smooth and glabrous ; fold and sinus faintly devel- 

 oped except at the anterior margins of the valves. 



The species embraced in this division have stronger differential characters 

 than are found among the preceding groups. The shells have a very short 

 hinge and low cardinal area, and the subcircular marginal outline causes a 

 noticeable alteration in the form of the spiral arms. These have their bases 

 well forward and are extended obliquely to the rounded cardinal extremities, 

 in their position thus approximating the form assumed by these organs in 

 Cyrtia and Cyrtina ; the crura, also, and the primary lamellae become very 

 long. This difi'erence is not, however, one of great significance and is to be 

 expected in any Spirifer having such an outline. 



The character of the muscular impressions is of greater importance ; the 

 broad scars of the diductors in the pedicle-valve are here reduced to very 

 narrow dimensions, are scarcely depressed and frequently not defined, but 

 represented only by a radiate marking of the shell. In the brachial valve the 

 adductor scars are two narrow impressions which widen anteriorly but are not 

 divided ti-ansversely. The surface of the shell was covered with very fine 

 concentric lines and the epidermal layer which is usually effaced, was minutely 

 punctate. Faint lateral plications are sometimes visible. 



These differences from the normal type of Spirifer have led many writers 

 to adopt McCoy's term Martinia for S. glaber and its allies. It is evident, 

 however, that this division of the smooth-shelled species embraces more than 

 one subordinate type of structure ; they may divided into 



1. Aseptati (= Martinia, McCoy, 1844). Shells in which dental lamellae and 

 septa are wanting. 



* There is a large ami hitherto uinlescrihed representative of this f,n-oiip in the limestone at Littleton, 

 Iowa, which is regarded by Piofessor Calvin as of upper Devonian ag-e. 



