FUNCTION OF APPENDAGES. 73 



3. What little can be learned of the musculature (see under musculature, seq.) indi- 

 cates that the trilobites had powerful extensor and flexor muscles, such as would be required 

 for this method of swimming. It may be objected that the longitudinal muscles were too 

 small to permit the use of a caudal fin. In the lobster, where this method of progression 

 is most highly developed, there is a large mass of muscular tissue which nearly fills the pos- 

 terior segments. Trilobites have not usually been thought of as powerfully muscled, but it 

 may be noted that in many cases broad axial lobes accompany large pygidia. As the chief 

 digestive region appears to have been at the anterior end, and other organs are not largely 

 developed, it seems probable that the great enlargment of the axial lobe was to accommo- 

 date the increased muscles necessary to properly operate the pygidium. It may be noted that 

 in all these genera the axial lobe of the pygidium is either short or narrow. 



4. The geological history of the rise of caudalization favors this view. With the ex- 

 ception of the Agnostidae and Eodiscidse, all Lower Cambrian trilobites had small pygidia, 

 and the same is true of those of the Middle Cambrian of the Atlantic realm (except for the 

 Dorypyge of Bornholm). In Pacific seas, however, large-tailed trilobites of the families 

 Oryctocephalidae, Bathyuridae, and Asaphidse then began to be fairly common, though mak- 

 ing up but a small part of the total fauna of trilobites. In the Upper Cambrian of the 

 Atlantic province the Agnostidae were the sole representatives of the isopygous trilobites, 

 while in the Pacific still another family, the Dikelocephalidae, was added to those previously 

 existing. 



With the Ordovician, caudalization reached its climax and the fashion swept all over 

 the world. It is shown not so much in the proportion of families with large pygidia, as in 

 the very great development of the particular trilobites so equipped. Asaphidae and Illaenidse 

 were then dominant, and the Proetidas, Cyclopygidae, Goldiidae, and Lichadidae had begun 

 their existence. A similar story is told by the Silurian record, except that the burden of 

 the Asaphidse has been transferred to the Lichadidae and Goldiidas. All the really old (Cam- 

 brian) families of trilobites with small pygidia had now disappeared. In the general dwin- 

 dling of the subclass through the Devonian and later Palaeozoic, the few surviving species 

 with small pygidia were the first to go, and the proetids with large abdominal shields the 

 last. 



The explanation of this history is probably to be found in the rise of the predatory 

 cephalopods and fishes, the natural enemies of the trilobites, against whom they could have 

 no other protection than their agility in escaping. While the records at present known carry 

 the fishes back only so far as the Ordovician (fishes may have arisen in fresh waters and 

 have gone to sea in a limited way in the Ordovician and more so in Silurian time) and the 

 cephalopods to the Upper Cambrian, the rise of the latter must have begun at an earlier 

 date, and it is probably no more than fair to conjecture that the attempt to escape swim- 

 ming enemies caused an increase in the swimming powers of the trilobites themselves. At 

 any rate, the time of the great development of the straight cephalopods coincided with the 

 time of greatest development of caudalization; both were initiated in the Pacific realm, and 

 both spread throughout the marine world during the middle Ordovician. And since, in the 

 asaphids, a decrease in swimming power of the appendages accompanied the increase in the 

 size of the pygidium, it seems probable that the swimming function of the one had been 

 transferred to the other. A high-speed, erratic motion which could be produced by the 

 sudden flap of a pygidium would be of more service in escape than any amount of steady 

 swiftness produced by the oar-like appendages of an animal of the shape of a trilobite. 



