152 Report oF THE STATE GEOLOGIST. 
thus forming an accessory articulation of the valves. The essen- 
tial articulation is effected by means of stout, simple teeth lying 
at the marginal extremities of the triangular deltidial covering 
and supported by dental plates which are usually short, but, in 
rare types, may be produced even to the anterior margin of the 
valve. The pedicle-passage or delthyrium is usually open. Nor- 
mally it is closed by a pair of deltidial plates having the form of 
scalene triangles, which develop from the sides of the delthyrium 
and meeting, inclose wholly or partially a circular or oval pedicle 
foramen. At normal maturity these plates become anchylosed 
along the median suture and form a single convex plate (the so- 
called pseudodeltidium). 
The usual absence of the deltidial covering may be due either 
to accidental removal or to resorption with advancing growth. 
In the adult and senile stages of development many species, 
especially in the line of development to Syrineorsyris, form a 
testaceous callosity in the pedicle-cavity, thickening the umbo 
and extending across the delthyrium, reaching, in extreme cases, 
nearly to the cardinal margin. 
The muscular area consists of a subtriangular pedicle-impres- 
sion occupying the pedicle-cavity, and continuous with a deeply 
impressed oval or obcordate area, which is posteriorly situated 
and divisible into a narrow median adductors and broad lateral 
diductors, the surface of the latter being marked by radiating or 
racemose furrows. The posterior and anterior members of the 
diductors may frequently be distinguished, the former being of 
less extent and their surface markings somewhat different from 
those of the latter. 
A median septum in this valve is usually absent, though not 
infrequently it is in a condition of incipient development, and in 
certain species having the aspect of Sprrirermna, it forms a most 
conspicuous feature of the interior. 
In the brachial valve the umbo is inconspicuous ; a median 
fold corresponds to the sinus of the opposite valve. The cardinal 
area is narrow, and divided by a broadly triangular delthyrium. 
The dental sockets are narrow, moderately deep and bounded 
interiorly by highly developed socket walls, the extremities of 
which support the crural bases. 
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