2 W. E. ALKINS — Morphogenesis of Brachiopoda 



obtained; each rose abruptly, and fell off rather less abruptly, 

 showing a certain correlation of the different variants. The 

 correlation between number of plications in the sinus and number 

 of plications on the whole valve, stated in a precise form by Hall, 

 was found to be approximate rather than precise. Extreme forms 

 were small in size, and conversely forms near the modes were 

 large and robust. Finally, there was absolutely no character or 

 combination of characters that could be relied upon to separate 

 any large collection into distinct species, though several such 

 species or varieties had been described; these forms to a limited 

 extent differed in range. 



No actual measurements were published. It is perhaps open 

 to criticism that the whole series of shells was not? taken from a 

 carefully restricted area. 



Day measured a thousand examples of Reticularia lineata 

 (Martin) from " one spot in the limestone near Peakshill Farm, 

 Rushup Edge Valley, Castleton." Remarkable variation in the 



form of individual shells was found, but distribution curves for 



Length , Length . ,. . r . 



the ratios - — ^-r and _ , gave no indication or the presence 

 Breadth Depth fo v 



of two or more forms. Each of the ratios investigated was found 

 to decrease as the shells increased in size, and the degree of 

 correlation between variation in breadth and variation in depth 

 was found to be equal to about one fifth of the whole variation; 

 apart from this, the variations in breadth and depth occurred 

 quite independently of one another. 



Again, no detailed measurements were published. The varia- 



, Length , Length . . . ,. 



tion in the - — ^-r and _ " ratios as the shells increase in 

 Breadth Depth 



size was only indicated qualitatively, without any attempt at a 

 quantitative treatment. Fortunately Day's series of shells is pre- 

 served in the Manchester Museum, and the writer has submitted 

 them to the method of investigation described below. 



Mook investigated five mutations of Spirifer mucronatus, 

 Conrad, from the Hamilton beds near Thedford, Ontario, and 

 near Alpena and at other localities in Michigan. In the case of 

 each of the mutations studied — mut. alpenense, Grabau ms. ; mut. 

 multiplicatus, Grabau ms. ; mut. profundus, Grabau ms. ; mut. 

 thedfordense, Shimer and Grabau; and mut. attenuatus, Grabau 

 ms. — distribution curves were plotted for adult and neanic shell 



indices, i.e., — r ratios. From the results it was concluded 



Length 



that alpenense is ancestral to profundus and thedfordense, whilst 



