72 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



of the thorax and pygidium in Triarthrus, Cryptolithus, 'and Acidaspis probably show an 

 adaptation of the endopodites of the posterior part of the body both as more efficient push- 

 ing organs and as better swimming legs. The fact that these segments are pointed below 

 enabled them to get a better grip on whatever they were crawling over, and the flatten- 

 ing allowed a much greater surface to be opposed to the water in swimming. In this con- 

 nection it might be stated that it seems very probable that the trilobites with large pygidia 

 at least, perhaps all trilobites, had longitudinal muscles which allowed them to swim by an 

 up and down motion of the fin-like posterior shield, the pygidium acting like the caudal fin 

 of a squid. Such a use would explain the function of the large, nearly flat pygidia seen 

 in so many of the trilobites beginning with the Middle Cambrian, and of those with wide 

 concave borders. It should be noted here that it is in trilobites like Isotclus, with pygidia 

 particularly adapted to this method of swimming, that the endopodites are most feebly de- 

 veloped, and show no flattening or modification as swimming organs. 



The relatively strong, curved, bristle-studded endopodites of Cryptolithus, combined 

 with its shovel-shaped cephalon, indicate Limulus-\ike burrowing habits for the animal, and 

 the mud-filled casts of its intestine corroborate this view. That it was not, however, en- 

 tirely a mud groveller is indicated by its widespread distribution in middle Ordovician times. 



Use of the Pygidium in Swimming. 



The idea that the use of the pygidium as a swimming organ is a possible explanation 

 of that caudalization which is so characteristic of trilobites has not been developed so far 

 as its merits seem to deserve. Two principal uses for a large pygidium of course occur to 

 one : either it might form a sort of operculum to complete the protection when the trilo- 

 bite was enrolled, or it might serve as a swimming organ. That the former was one of its 

 important functions is shown by the nicety with which the cephalon and pygidium are 

 adapted to one another in such families as the Agnostid;e, Asaphida;, Phacopidse, and others. 

 That a large pygidium is not essential to perfect protection on enrollment is shown by 

 an equally perfect adjustment of the two shields in some families with small pygidia, notably 

 the Harpedida; and Cheiruridas. That the large pygidial shields are not for protective pur- 

 poses only is also shown by those forms with large pygidia which are not adjusted to the 

 conformation of the cephalon, as in the Goldiidse and Lichadidre. It is evident that a large 

 pygidium, while useful to complete protection on -enrollment, is not essential. 



It would probably be impossible to demonstrate that the trilobites used the pygidium 

 in swimming. The following facts may, however, be brought forward as indicating that they 

 probably did so use them. 



1. The appendages, both exopodites and endopodites, are relatively feebly developed 

 as swimming organs. This has been discussed above, and need not be repeated. It must 

 in fairness be observed, however, that many modern Crustacea get about very well with 

 limbs no better adapted for swimming than those of the trilobites. 



2. The articulations of the thoracic segments with each other and with the two shields 

 are such as to allow the pygidium to swing through an arc of at least 270, that is, from 

 a position above the body and at right angles to it, around to the plane of the bottom 

 of the cephalon. Specimens are occasionally found in which the thorax and pygidium are 

 so flexed that the latter shield stands straight above the body. A well preserved Dipleura 

 in this position is on exhibition in the Museum of Comparative Zoology, and Mr. Narraway 

 and I have figured a Bumastus milleri in the same attitude (Ann. Carnegie Mus., vol. 4, 

 1908, pi. 62, fig. 3). 



