474 



Forty-fifth Report on the State Museum. 



Fig. 32.— Proboscidella 

 proboscideus (Ethe- 

 ridge). 



In many of these shells the pedicle-valve was provided with 

 spines serving to attach it more firmly to its place 

 in the sediment, so that it became less liable to 

 be washed from its position ; and among such 

 spiniferous shells we find the greatest convexity 

 of this valve, and often a remarkable extension 

 of its anterior margins (Productus). In Pro- 

 ductus {Proboscidella) proboscideus and P. 

 JVystianus, this modification, is carried to an 

 extreme, the pedicle- valve being greatly produced 

 at its free margins which meet on the lower side 

 and form/a long tube, while the brachial valve is 

 small, unmodified and operculiform. It has been 

 suggested that the function of this modification 

 is to transmit water to the animal deeply buried 

 in the sediments. 



An opposite extreme of form occurs in certain 

 nearly plane species of Strop seodonta (Lepto- 

 strophia ; plate 15, figs. 1-4), in which the two valves have main- 

 tained an equable growth for their entire extent. 



There are a few shells of this concavo-convex group, 

 whose valves have a double curvature. Thus in 

 Strophomena, STROPHONELLAandCHONosTROPHiA, the 

 contour of the two valves in youth, or 

 in the umbonal region of adult shells, 

 is normal, the pedicle-valve being .. 

 convex, the brachial concave ; but in, 

 later growth this contour becomes FlG 34 --^<>J>>"> 



o mena Wisconsin- 



Fia.3s.—stropho- reversed, and the general concavity emis. Proaieview 



mena deltoidea. » ,1 in* ±\ „ -.-^^-.'^i -> , T ,K T ^ showing the con- 



of the shell is on the pedicle-valve vexity ^ the bra- 

 and its convexity on the brachial valve. The chiaivaiveandth© 

 causes of such reversion in contour can only be ped^vaive ^ 

 due to an accelerated growth of the brachial valve the umbonai 

 after the early stages have been passed; and it 

 may be observed that species of Strophonella are all, virtu- 

 ally, reversed Stropheodontas. Stropheodonta is a genus 

 which in its adult state, at successive periods in its history, 

 exhibits in a most remarkable manner various phyletic develop- 

 mental phases. Thus its earlier (Silurian) representatives 



26 



