BIOLOGICAL PALAEONTOLOGY 105 



efficiency. Among Mollusca, the Pelecypod family of 

 the Pernidae produced, in Upper Cretaceous times (when 

 the group was on the wane), species of Inoceramus that 

 attained a diameter of about two feet, and secreted shells 

 of a massiveness disproportionate even to that great 

 size. Among Ammonites, such a name as Pachydiscus 

 peramplus is sufficiently descriptive of comparable 

 development towards the end of the life of the class. 

 Excessive calcification is equally characteristic of " end- 

 forms " in various groups. It may be shown in two 

 ways, which seem, in some measure, antithetic. In 

 some cases the superfluous secretion appears as warty 

 or spinous outgrowths from shells whose ancestral types 

 were relatively smooth. Productus (PI. xi. fig. 6) 

 illustrates this quality among the Brachiopods, while 

 many species of Spondylus (particularly recent forms) 

 may probably be claimed as examples of the same 

 tendency. In other cases the additional mineral matter 

 is spread uniformly over the outer surface of a shell, 

 increasing its thickness and obliterating the ornament 

 that characterized earlier members of the group. This 

 " gerontic laevigation " is well shown in the later species 

 of Parkinsonia among Ammonites, where the strongly 

 ribbed character normal to the series is lost, in the outer 

 whorls at least, in the massive shells of such types as 

 P. dorsetensis (PL viii. fig. l). Clavella (PI. viii. figs. 2 

 and 3) shows a corresponding development among Fusid 

 Gastropods. The aberrant types Richthofenia and 

 Hippurites perhaps represent comparable phases in 

 the degeneration of Brachiopod and Pelecypod stocks 

 respectively. 



There appear, then, to be two distinct methods where- 

 by races of organisms may suffer extinction. In the re- 

 gressive quality there seems to be a suggestion of some 

 tendency akin to that of senescence ; the group attains 



