No. 444-] PTE RASP IDA*: AND CEPHALASPIDAE. 



That the Cephalaspidae were sluggish animals is indicated not 

 only by the clumsy shape, and large size of the heavily armored 

 head, but also by the absence of an axial skeleton, and by the 

 feeble development of the trunk and the dorsal and caudal fins. 



The overlapping of the large scales on the dorsal surface of 

 the trunk in Cephalaspis, and the presence of minute ventral 

 scales, indicate some freedom of movement in a dorso-ventral 

 direction and a restriction of those lateral movements so essen- 

 tial to sustained swimming when well developed appendages for 

 that purpose are absent. 



The dorsal fin was short and low and covered with close set 

 scales that would allow but little freedom of movement. 



The Cephalaspidae, therefore, were in all probability bottom 

 feeders. Any one that has watched Limuli, both old and young, 

 ploughing slowly through the soft mud and sand, leaving little 

 more than their median ocelli and lateral eyes exposed, could 

 hardly avoid the conclusion that many of the trilobites and Ceph- 

 alaspidae, whose eyes are placed high up on the convex surface 

 of similar shovel- shaped heads, must have moved about on the 

 bottom in a similar manner. 



But Limulus frequently crawls with considerable rapidity 

 over the surface of shallow bottoms, or turning on its back with 

 the aid of its caudal spine, even the largest and heaviest female 

 may leave the bottom and swim slowly away with legs, operculum 

 and gill covers beating the water in unison with oar like strokes. 

 The young larvae, especially in the trilobite stage, swim very 

 persistently in this inverted manner. It is also well known that 

 Branchipus, Apus and many copepods swim in this position, 

 and there can be but little doubt that many trilobites and 

 merostommata did the same. 



The Cephalaspidae were certainly disproportionately heavy at 

 the anterior end, so that any attempts to swim by movements 

 of the trunk alone would tend to push the head deeper into the 

 mud or sand, a tendency that would be increased by the sharp 

 downward slope of the anterior dorsal surface. It is also proba- 

 ble, judging from the shape of the head and trunk, that its 

 centre of gravity was situated above the mass centre, so that if 

 the animal did succeed in getting off the bottom, it would have a 



