Il6 NEW YORK STATE MUSEUM 



culminating or just beyond their climacteric period, the percentage 

 is too high, the great number of recent genera not being taken into 

 account. 



Immortal Types 



Of greatest interest are the types which range from the Paleozoic 

 to the present time. These, one might well term immortal types. 

 They are principally found among the Foraminifera (n genera 

 and 13 from the Triassic-Recent) ; Pelecypoda (9 genera) ; Gas- 

 tropoda (13 genera and 12 genera from Triassic-Recent) ; and 

 Ostracoda (7 genera). It is, however, to be considered that at 

 least some of these extreme ranges may result from as yet incom- 

 plete knowledge of the forms, especially among the Pelecypoda 

 and Gastropoda. Thus, of the two immortal bryozoans, Berenicea 

 and Stomatopora, the latter has lately been shown by Bassler to 

 differ from the Paleozoic forms hitherto referred to them, and in 

 the case of the gastropods, Perner restricts Pleurotomaria to the 

 Mesozoic genera, separates Platyceras from Capulus of Tertiary 

 and recent age, but points to the great resemblance of the 

 Paleozoic forms to this latter genus. 1 



Considering, however, the close study that has already been 

 given to the lower invertebrates both here and abroad, it does not 

 seem to us probable that these figures will be greatly reduced. On 

 the contrary, a comparison of the figures given in the preceding 

 table and taken from the editions of Zittel-Eastman of 1896 and 

 1913 even brings out a considerable increase in the more important 

 groups, namely, in the Foraminifera from 29 to 48, the Corals 

 from 42 to 46, the Bryozoa from 49 to 68, the Pelecypoda from 

 71 to 78, the Gastropoda from 108 to 126, all in the short period 

 of seven years, as the result of the discovery of longer ranges of 

 forms. 



But even if it be conceded for the sake of argument that the 

 majority of these persistent genera may, on further study, be 

 found to be still divisible into successive groups and that the abso- 

 lute persistence is hence merely a deception due either to lack of 

 knowledge or to extremely slow but still perceptible variation and 

 development, then the fact of their extremely slight change and 

 relative stability . as contrasted with the rapid development of the 



1 He cites as gastropod genera remarkable for their longevity : Carlnaropsis, 

 Clisospira, Euryzone, Hercynella, Palaeacmaea, Loxonema, Platyostoma, 

 Turbonitella and Eotomaria. The four genera Bembexia, Calloconus, Mour- 

 lonia and Trochonema are peculiar for their manner of reappearing after long 

 periods of absence (Geol. Mag., Aug. 191 1, p. 374) 



