758 Report oF THE State Gxrotoaist. 
Aperturati; their surface is frequently ornamented with fine 
granules or interrupted radiating stria. The cardinal process is 
developed as a broad, thin, spreading plate, crossed longitudin- 
ally by numerous linear depressions. A feature which appears 
at times in other groups, but which here possesses the highest 
significance, is the gradual development of the callosity or trans- 
verse plate in the apex of the delthyrium. Originally, and 
always in the earlier species (S. perextensus, Meek and Worthen, 
S. macrothyris, Hall, etc.), an accompaniment of adult or senile 
growth, it eventually becomes a permanent character existing 
throughout all the later immature growth-phases of the shell. 
In its simpler manifestations it is a testaceous deposit extending 
across the delthyrium from its inner margins; as its size increases 
it unites the dental lamelle, fills the rostral cavity of the valve 
and extends forward along the bottom of the shell between the 
posterior extremities of the diductor muscular bands. This is 
its condition as usually seen in the middle Devonian species, 
S. granulosus and 8. audaculus. 
Not infrequently this plate is less thickened and extends 
downward with a convex outer surface for two-thirds the length 
of the delthyrium, but this particular form of development 
occurs less often in the early species. 
In all its phases it may be coexistent with the true deltidium, 
though the latter is rarely retained in growth-stages where the 
apical callosity is well developed. 
Section VI. Grasrati. (Plate 29, figs. 1-3.) Typical examples, 
Spirifer glaber, Martin, Martiniopsis inflata, Waagen. 
Forms with the surface smooth and glabrous; fold and sinus 
faintly developed 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 Cyrira and Cyrrina; the crura, also, and the primary 
lamelle become very long. 
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