Manchester Memoirs , Vol. Ixiv. (1920), No. 2 



II. — Morphogenesis of Brachiopoda. 

 I. — Reticulata lineata (Martin), Carboniferous Limestone. 



By W. E. Alkins, M.Sc. 



( Communicated by Dr. George Hickling, F.G.S.) 



( Read December 2nd, 1919. Received for publication February iyth, 1920.) 



Introduction. 



In view of the great amount of biometric research that has 

 been carried out during the last few years, it is somewhat 

 surprising that very few investigations along such lines deal 

 with Brachiopoda. The group is represented at the present time 

 by rather less than a hundred and forty species, few of which 

 occur in conditions such as would make them accessible in 

 sufficient numbers for purposes of measurement; but brachiopods, 

 except in the Tertiary rocks, constitute one of the most abundant 

 and most important groups of fossils. Interesting to the zoologist 

 not merely on account of the great age of the group, but also by 

 reason of the extreme variability often exhibited within a single 

 species, and of the prevalence of the phenomenon of heterogenetic 

 homceomorphy, they are of the highest importance to the strati- 

 grapher, since they occur almost world-wide, and frequently in 

 greater abundance than any other group. Hence an enquiry into 

 the variation and growth of a number of species may be of some 

 interest. 



Previous Research. 



Quantitative studies of variation in certain species of fossil 

 Brachiopoda have been published by Cumings and Mauck (1), 

 by Day (2), and by Mook (3). The first-named authors studied 

 Platystrophia lynx from the Upper Ordovician rocks at Vevay, 

 Indiana (Switzerland Co.). The data taken for study were: 

 Ratio of width to length of shell; ratio of depth to breadth of 

 sinus; number of plications on ventral valve; number of plica- 

 tions on dorsal valve; number of plications in sinus; number of 

 plications on fold. In the case of each of these quantities a 

 fairly regular, though slightly asymmetric, distribution curve was 



December joth, ip20. 



