SPIRIFER THEDFORDENSIS S. AND G. *173 



In the interior of the pedicle valve is usually found a faint median 

 septum which divides the muscular area; this, though not so strong as 

 that of Sp. consobrinus, is nevertheless of the same type, and appears to 

 mark the relationship between that species and the variety thedfordensis. 



In the following table measurements of three stages in development 

 of this variety — that is, the early or nepionic* the youthful or manic, 

 and the adult or ephebic — are given, the actual height and width being 

 first considered and after that the proportional width, the height being 

 taken as unity. The nepionic is considered to be the stage when the 

 shell is non-mucronate, the measurement being taken at the last line of 

 growth preceding the mucronation. The neanic stage is the mucronate 

 stage, the measurements being made at the stage of extreme mucrona- 

 tion. The ephebic is the measurement of the adult. 



The average proportional width of the adult ranges from 1.5 to 2. A 

 few individuals fall below that, and these represent extreme acceleration. 

 They are numbers 19, 22, 30, and 56. Number 19 is the most acceler- 

 ated of these, the proportional width of the adult being 1.2, while that 

 of the neanic is as low as 1.96, showing that the adult characters were 

 early acquired, the mucronate stage being relatively short. The speci- 

 men is a short, rotund individual. 



Examples of unusual retardation are found in numbers 2, 6, 7, 10, 16, 

 17, 38, 49, 50, and 59, or in all those in which the proportional width is 

 above 2. Eliminating both accelerated and retarded individuals, we get 

 an average proportional width for the remaining of 1.85. Number 16, 

 with a proportional width of 2.51, and number 38, with a proportional 

 width of 2.4, represent extremes of retardation. They are comparable 

 to accelerated individuals of the normal S. mucronatus. Their extrem- 

 ities are extremely acute, but not mucronate. Number 14 is a young 

 individual. 



From an inspection of the tables it appears that the average propor- 

 tional width for the neanic stage ranges from 3.2 to 4.2, nearly 75 per 

 cent falling within this limit. Of these the average proportional width 

 is 3.8. The following thirteen are strongly accelerated in this stage : 

 Numbers 5, 12, 15, 19, 22, 28, 30, 36, 39, 43, 47, 55, and 56. Of these 

 number 19 is the most accelerated, as already noted, having a propor- 

 tional width of only 1.96 in this stage. Examples of unusual retardation 

 in this stage are seen in numbers 9, 18, 26, 48, and 57. Of these number 

 18 is the most retarded, with a proportional width of 4.9, and numbers 

 48 and 57 are next, with a proportional width of 4.7 each. The average 

 proportional width of 6 normal adult S. mucronatus from Michigan was 



* For this terminology see papers by Hyatt, Jackson, Beecher, and others. 

 XXVI— Bull. Geol. Soc. Am., Vol. 13, 1901 



