172 SHIMER AND GRABAU — HAMILTON GROUP OF THEDFORD 



While in variety arkonensis of the lower beds this condition persists 

 for a long time, thus producing the elongated form, in var. thedfordemis 

 the mucronate stage is comparatively short. After a certain time the 

 shell no longer increases in width to an appreciable amount, but ad- 

 dition is chiefly made to the front, so that the frontal margin on either 

 side of the center changes from a concave to a straight and finally convex 

 outline, the shell at the same time increasing in height and convexity. 

 Since the additions are all made to the anterior margin, no new plications 

 appear, the number being about the same as that found on the young of 

 variety arkonensis of the same width or somewhat less. With the dis- 

 appearance of the mucronations in the young of variety thedfordensis, 

 the depression in the fold and incipient plication in the sinus also dis- 

 appear; the shell becomes extremely robust, the fold and sinus as well 

 as plications and concentric lamellae becoming strongly pronounced. 

 This, together with the unusual abundance of this variety in these upper 

 beds, indicates that the species had become well adapted to its environ- 

 ment. Even within this variety there is considerable variation ; this 

 in one case is due to extreme acceleration which produces a transversely 

 short and thick-set form, the mucronate stage having been passed through 

 rapidly and the number of plications as a result being small. On the 

 other hand, a retarded development would cause a prolonged mucronate 

 stage which gives the shell an extended character with acute, or in some 

 cases of extreme retardation, slightly mucronate extremities. In these 

 extended individuals the number of plications on either side of the fold 

 or sinus ranges from 15 to 20, while, on the other hand, in the short- 

 winged forms the number is not over 10 and sometimes even less. In 

 the long-winged varieties the flattening or median depression of the fold 

 is generally somewhat longer retained than in the short- winged ones, as 

 might be expected. Occasionally the mucronate stage becomes extremely 

 condensed, through acceleration in development, so as to be almost en- 

 tirely eliminated ; no specimen, however, in which this stage was alto- 

 gether eliminated has been observed. A few specimens showing old age 

 features have been found ; in these new lamellae have been added; 

 which, however, scarcely extend beyond the frontal margin of the adult. 

 Thus no further increase in height is effected, but a great thickening 

 of the anterior margin, marked by numerous lamellae, is produced. In 

 the New York Hamilton an accelerated variety of Spirifer mucronatus^ 

 which simulates in form the Thedford species, is very common ; in this 

 the early stages are very similar to the Thedford variety, but the plicated 

 sinus and sinuate fold are in general retained throughout, thus indicat- 

 ing a less degree of acceleration. The New York variety, too, is less 

 robust, and the concentric lamellae are less strongly marked and less 

 regular. 



