TRENTON FOSSILS. 293 



The specimens of Crania (?}filosa collected in Vermont are from South Hero. 



Discina. The shell of Discina is orbicular, horny; the upper valve limpet-like, smooth or concentrically 

 lamellose, apex behind the center; the lower valve flat or conical, with a sunk and perforated disk on the 

 posterior side; interior polished; lower valve with a central prominence in front of the foramen. 



The shell of the Discina lamellosa (Hall), is orbicular, depressed; apex small, but little elevated, situ- 

 ated about one-third the breadth of the shell from the margin; surface marked by elevated lamellifonn 

 concentric lines or ridges. 



The shell is nearly flat, with the exception of the small apex. The concentric lines appear to be made 

 up of their shelly lamines. 



Fig. 197 represents a ventral view of the Discina lamellosa. It is from McNeil's Point, in Charlotte, 

 where so many brachiopod shells are found. It is a rare species. 



Trematis. In this genus the shell is depressed; valves unequally convex, greatest in the lower 

 valve; umbo of the lower valve sub-central; of the upper marginal; lower furnished with an oblong 

 aperture for the transmission of the peduncle. 



The Trematis terminalis (Em.) is the only species of this genus in Vermont, and is quite com- 

 mon at Larrabee's Point, in Shoreham, and at McNeil's Point, in Charlotte. The shell is very 

 obtusely subovate or orbicular; ventral valve depressed, convex, with a terminal or marginal apex; dorsal 

 valve very convex or subconical, with a central apex, a broad depression and narrow slit on one side, ex- 

 tending to the margin of the shell opposite the beak of the ventral valve; surface usually smooth, perhaps 

 from exfoliation. Fine longitudinal or radiating stria? are also visible when the shell is exfoliated; these 

 strife are sometimes punctate. 



Tin's fossil is easily recognized by its low, convex suborbicular ventral valve, with a marginal apex. The 

 opposite valve is less often seen, but it is as readily distinguished by the broad depression on one 



IG ' side of the apex, with the narrow sli t in the bottom for the protrusion of the peduncle. 



Fig. 198 represents a ventral valve of the Tremalis terminalis with a marginal apex. 

 Strophomena. The shell Strophomena is semi-circular, widest at the hinge line, concavo- 

 convex depressed, radiately striated; area double; ventral valve with an angular notch, progress- 

 ively covered by a convex pseudo-deltidium ; umbo depressed, rarely (?) perforated, in young 

 shells, by a minute foramen; muscular depressions four, central pair narrow, formed by the adductor; ex- 

 ternal pair fan-like, left by the cardinal and pedicle muscles; dorsal valve with a bi-lobed cardinal process, 

 between the dental sockets, and four depressions for the adductor muscles. 



There are no apparent brachial processes in the dorsal valve of Strophomena, and it is possible that the 

 spiral arms may have been supported at some point near the center of the shell. The valves are nearly flat 

 until they approach their full growth; then they bend abruptly to one side. 



Strophomena alternata (Con.) is broadly semi-oval; length and breadth about as 12 to 15; hinge line, 

 in perfect specimens, a little longer than the width of the shell, slightly reflected at the extremities, which 

 sometimes become short acute ears ; cardinal area narrow, the callosity of the ventral valve nearly filling 

 the triangular foramen of the dorsal valve ; beak uniformly perforated with a minute circular opening; 

 dorsal valve depressed convex, sometimes more convex in the middle, suddenly deflected near the margin 

 and flattened towards the cardinal line; ventral valve concave, gradually, or sometimes suddenly inflected 

 towards the basal margin; surface marked by fine rounded radiating strife, which alternate at unequal 

 intervals with coarser ones; strife increasing in number towards the margin of the shell, crossed by fine 

 elevated concentric lines and a few imbricating lines of growth. 



In this species the stria? are usually of two sizes, the coarser and more elevated ones having from four to 

 six finer ones between; the latter increase in number as they recede from the apex, and one of them, in the 

 center of the fascicle, becomes enlarged, and was above the others. The characteristic developments of 

 the shell depend on the circumstances of its existence, and the nature of the sediment. There are other 

 differences which must be due to other causes beyond our knowledge. The age of the shell also exerts 

 considerable influence upon its external form. The young ones, or those of medium size, are usually 

 symmetrical shells, moderately convex, and gently curving towards the base. The strife in such individuals 



