SPIRIFER MUCRONATUS VAR. ARKONENSIS 171 



In tracing out the successive stages in the development of this variety 

 by means of the growth lines, it becomes apparent that in the stage im- 

 mediately preceding the adult stage, individuals of this variety are 

 strongly mucronate ; this mucronation becoming obsolete only by the 

 addition of later lamellae in the adult stage. At a still earlier stage the 

 mucronation does not exist, the shell terminating in regular acute angles, 

 and still earlier in right or rarely obtuse angles. In the earliest stage 

 observed, the shell appears to have been striate, in addition to the few 

 plications, but these striae are seldom well preserved. In a few cases in 

 individuals which have almost the size of the adult, the mucronate 

 character of the earlier stage persists, this being due to a retarded devel- 

 opment. In these therefore the more primitive features are retained 

 throughout the life of the individual, which thus never passes beyond 

 the normal characters of the primitive type. Such primitive forms char- 

 acterize the lower Hamilton shales of northeastern Michigan, these 

 never acquiring the elongate, non-mucronate, and multiplicate stage 

 characteristic of the variety arkonensis. 



Throughout the Hamilton strata of New York individuals occur which 

 are mucronate in their adult stage ; these when not primitive, as is prob- 

 ably the case with those in the lower strata, are examples of a retarded 

 development, the retardation affecting the individual or group of indi- 

 viduals (placed, perhaps, under unfavorable conditions) and not the 

 species. 



SPIRIFER MUCRONATTJS VAR. THEDFORDENSIS VAR. NOV.* 



This variety is characteristic of the upper beds of the Hamilton group 

 in the Thedford region, being entirely unknown below the Encrinal 

 limestone. It is more than a local mutation, for it represents a distinct 

 and decided advance in development of the typical form of that species. 

 This is shown by a study of its development, for the shell passes through 

 a series of transformations, which include a stage in which the imma- 

 ture shell has all the characters of an adult form of the primitive S. 

 mucronatus. As in all varieties of this species, the earlier (nepionic) 

 stage is non-mucronate and with a few plications only. At a compara- 

 tively early stage the mucronations appear, increasing in strength until, 

 in the adolescent (neanic) stage, the shell is extremely mucronate and 

 the fold of the brachial valve has a distinct median depression, while 

 the flattening or emargination in the lines of growth in the median sinus 

 of the pedicel valve indicates the persistence during this stage of the 

 median plication characteristic of the primitive S. mucronatus. 



* Although this name is frequently used by collectors and students, we have been unable to find 

 it in print anywhere. 



