. . 



21. Chondrodonta Bo*ei; a new species of fossil Lamelli- 



branchs from the Hippurite-bearing beds of 



Seistan* 



By Ernest W. Vredenburg. 



The geological period known as the Cretaceous witnessed 

 an extraordinary development of certain ponderous marine 

 bivalves, in which the ordinary structure of a pelecypod shell has 

 became disguised to such an extent as to simulate frequently a 

 coralloid growth, while the valves are apt to become so unequal 

 in size as to bear to one another very much the same relation as 

 the shell and operculum of a gastropod. These singular or- 

 ganisms were for a long time a puzzle to naturalists, and it is 



only quite lately that the homology of their structure- has b< 

 to be clearly understood. They grew together in shallow water 

 forming large masses that recall a coral-reef much more than the 

 banks built by certain gregarious molluscs such as oysters. 

 This abnormal mode of development occurs in a number of enera 

 which are not always closely related to one another, and must 

 be referable therefore to certain biological conditions special 

 to the particular geological period during which these peculiaj 

 shells flourished. They reached the acme of their development 

 towards the close of the Mesozoic or secondary period which 

 not a single one survived. It is very remarkable to note that a 

 closely similar mode of growth affected an entirely different class 

 of organisms, that of the Brachiopods, at a previous period of 

 the earth's history, that is during th« final stages of the I' daeozoic 

 or Primary era. Both during the terminal stag* of the Paleo- 

 zoic and Mesozoic, there occurred an exceptional spreading of 



the oceans which overflowed all the low-lying part- of the con- 

 tinents, and considerably increased the area of the -hallow 

 portions of the sea floor which are best adapted to a luxurious 

 development of marine life. It is during these exceptionally 

 favourable periods that the peculiar types of permanently fixed 

 organisms above alluded to have mainly flourished, In both 

 instances, both at the end of the Palaeozoic and at the end of the 



Mesozoic these periods of oceanic extension were followed by a 

 sudden regression during which the sea-level sank far below 

 present limits and the great sub-marine continental ledges 



became dry land for a short time. To this sudden alteration 

 of physical conditions is to be ascribed, in all probability, the 

 abrupt changes in the organic world that marked the initial 

 phases of the secondary and of the Tertiary eras. 



