4 W. E. ALKINS — Morphogenesis of Brachiopoda 



The Present Research. 



In the present paper the writer proposes to give the results of 

 a study of the series of Reticularia lineata (Martin) measured 

 by Day. The specimens preserved in the Manchester Museum 

 number nine hundred and forty-five (Day measured one thousand). 

 Day did not accurately define the position of the three axes, 

 Length, Breadth, and Depth, measured by him, and therefore no 

 attempt had been made to discover whether the results of the 

 measurement of the same series of shells by different observers 



were in accordance. Indeed, the values of the two ratios - 



and - — *— _ have not been determined, since the present investiga- 

 tion was undertaken with the object of studying the growth rather 

 than the variation of the species. 



The history of the species was sketched by Day, who also 

 dealt at length with variations in certain characters less susceptible 

 of measurement — variation in size, and lateral twisting of the 

 ventral beak, ornament, etc. — as well as with variations in form. 

 It is therefore unnecessary to treat of these features here. 



Measurement. The three axes Length, Width, and Depth, 

 were measured by means of an optician's sliding gauge, the 

 determination being in every case correct to the nearest millimetre. 

 The position of the axes may be defined as follows : — 



Le?igth. From the brachial umbo to the anterior margin, 

 along the plane of symmetry of the shell. By measuring from 

 the brachial umbo, irregularities due to variation in the relative 

 size of the umbones — which are much greater in the case of the 

 pedicle (ventral) umbo — are considerably diminished. 



Width. The greatest width, perpendicular to the length axis 

 and to the plane of symmetry. 



Depth. The greatest thickness of the shell, perpendicular to 

 the axes of width and length. 



All shells of the same length were now got together, the whole 

 series being thus divided up into a number of smaller series, 

 each comprising shells which had attained the same length. In 

 each of these small series width and depth distribution curves 

 were plotted, and these were utilised to construct skeleton solid 

 figures, in which the horizontal axes represent (i) length and (2) 

 width or depth, the vertical axis representing the number of 

 specimens The data on which these figures are based are given 

 in Tables I and II, while photographs of the skeleton figures are 

 shown in Figs. 1 and 2; the method adopted will probably be 

 evident from Tables I and II (in constructing the solid figures 

 outlying specimens have been disregarded). 



