822 LIFE : OUTLINES OF GENERAL BIOLOGY 



may be said of other cases where the evolutionary lineage is well 

 known, as of horses, elephants, and camels, each illustrating the 

 transformation of a race with a survival of efficient living repre- 

 sentatives. Yet there are many cases where a large group has become 

 wholly extinct, leaving no direct descendants, as is notably the case 

 with the probably pelagic Graptolites, which were so common in 

 Silurian seas. They are generally regarded as related to the Hydrozoa 

 of to-day, but they cannot be referred to any division of that large 

 and diverse class. Some of them were probably slow swimmers, 

 while others were attached to the drifting seaweeds of a Silurian 

 Sargasso Sea. But the point is that Graptolites represent a "lost 



Fig. 140. 



A Trilobite, an extinct type of Arthropods, lasting from the Cambrian to 

 the Permian R, rostrum or beak; A, antenna; HS, head-shield; 

 E, eye; AP, projecting appendages; TH, thorax; L, thoracic limbs; 

 P, tail-piece or pygidium. Alter Beecher. 



race"; they became extinct in the Lower Carboniferous, and they 

 are not known to have any persisting relatives. 



Similarly, the ancient Trilobites (Cambrian to Permian), their 

 allies the Eurypterids (Silurian to Permian), the two classes of 

 Echinoderms known as Cystoids and Blastoids, and the whole race 

 of Ammonites, are all lost races. The series of Nautiloids, parallel 

 to that of Ammonites, but not nearly related, is represented to-day 

 by the Pearly Nautilus, one genus with only four species. 



So among backboned animals there are four or five orders of 

 ancient reptiles that have no living representatives; and while it 

 may be objected that birds and mammals seem to have evolved 

 from two stocks of extinct Dinosaurs, whose blood has therefore 

 not ceased to flow, it is only necessary to mention the Pterodactyls, 



