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 Agnostida: and Eodiscidje, 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, Bathyuridre, 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 Agnostida; were the sole representatives of the isopygous trilobites, 
while in the Pacific still another family, the Dikelocephalida;, 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. Asaphidas and Illaenidae 
were then dominant, and the Proetidse, Cyclopygidas, Goldiida;, and Lichadidae had begun 
their existence. A similar story is told by the Silurian record, except that the burden of 
the Asaphidas has been transferred to the Lichadida? and Goldiidae. 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. 
