170 SHIMER AND GRABAU HAMILTON GROUP OF THEDFORD 



slight and augments the thickness of the shell rather than its width. 

 Thus an old age individual may well have an elongated form without 

 increasing much in width, and at the same time become bulging and 

 the beak sufficiently incurved to conceal the foramen. 



While L. laura is chiefly restricted to the shale layers within the 

 Encrinal limestone, a few representatives occur higher up in the upper 

 shales. Two specimens from Thedford agree essentially with Billings' 

 figure — the plications are numerous and continue to near the beak, and 

 the width is approximately equal to the height. The prevailing form 

 in the upper beds appears to be L. huronensis, which might be considered 

 as a passage form to the pauciplicate Upper Devonian species. 



SPIRIFER MUCRONATUS VAR. ARKONENSIS VAR. NOV. 



This well marked variety characterizes the lowest beds exposed at 

 Bartlett's mills. It is extremely elongated laterally, reaching a width 

 ranging to 70 or 75 millimeters, with a corresponding height of 15 to 18 

 millimeters. A typical individual measures 63 millimeters in width by 

 14 millimeters in height. Another measured 56 or 58 millimeters in 

 width by 16 in height. The wings, though much attenuated, are not 

 mucronate in the adult stage, but the frontal margin forms a straight 

 line from the fold or sinus to the extremity. The umbonal portion of 

 the pedicle valve is but slightly elevated above the hinge line. The 

 hinge area in some of the larger specimens has a height of from 2 to 2£ 

 millimeters at the delthyrium, but gradually narrows toward the ex- 

 tremities. The hinge line of the brachial valve is linear, and extends 

 the entire width of the shell. The median fold invariably bears a de- 

 pression, which is often quite strongly marked, and in rare instances 

 is only a flattening. Corresponding to this is a more or less strongly 

 marked median plication in the sinus, though at times this is merely 

 indicated by a slight emargination of the lines of growth. The plica- 

 tions are numerous and strong, decreasing very gently in width toward 

 the lateral extremities. In the adult there are 30 or more on each side 

 of the fold and sinus. They are rounded at the summit, and are sepa- 

 rated by narrower interspaces. Concentric lines of growth are quite 

 strongly marked, and are somewhat inequally distant, and generally 

 most pronounced in the interspaces. Toward the cardinal line all the 

 plications become obsolete, except those margining the fold and sinus. 

 The non-plicate portion along the hinge line is generally marked by fine 

 longitudinal strise, which are very numerous and somewhat irregular. 

 The cardinal process is strongly and regularly striated. The median 

 septum of the brachial valve is moderately developed, and a faint septum 

 occurs in the pedicle valve. 



