456 



Forty- fifth Report on the State Museum. 



the apex of the larger valve. In certain genera, however, fixa- 

 tion is attained by the cementation of the lower valve (Crania), 

 or of the umbonal portion of the larger valve (Strophalosia, David- 

 sonia, Lept^nisca, Chonopectus, etc.) to the external object. 



Fig. l.— Derbya Bennetti. An adult example in which the scar of attachment (x) is retained. 

 A Crania is attached to the umbo of the brachial valve. 



The former mode of attachment takes place at an early stage in 

 the growth of the animal, and is continued in some instances 

 throughout the remainder of its life (Terebratella, Magellania, 

 Hemithyris, etc.) ; at other times the pedicle becomes atrophied 

 after adolescence or maturity, by the incurvature of the beak of 



Fig. 2.— A portion of the inner surface of a brachial valve of Product us, to which a number of 

 very young shells of Derbya are attached by the cementation of the pedicle-valve. It will be 

 observed that most of the attached shells have the hinge line parallel to that of the Produc- 

 tus, though their relative positions are inverted. 



the larger valve, or by the closing of the aperture for its pas- 

 sage by adventitious deposits of calcareous matter, and the shell 

 thus becomes free. This condition is true of some living genera, 

 while it obtained among the majority o\' Palaeozoic species, 



8 



