1910] The Ottawa Naturalist. 133 



that the appendages under the pygidium of Triarthus were 

 flattened and adapted for swimming, while those under the 

 thorax could be used either in swimming or crawling. It would 

 seem that this sort of specialization had gone still further in 

 Isotelus than in Triarthus. Here there is a large pygidium, 

 probably provided with swimming organs (Walcott found traces 

 of them under the pygidium of the specimen he described), and 

 under the thorax the inner portions of the appendages were 

 strengthened to function as ambulatory appendages. In Tri- 

 arthus, a form adapted primarily for swimming and secondarily 

 for crawHng, the appendages are very long, and extend far 

 beyond the outer margins of the test, while in Isotelus the 

 appendages are hardly long enough to reach the outer margins. 

 The swimming power had to some extent been sacrified in the 

 adaptation to crawling. Triarthus depended on Its swiftness in 

 swimming to escape from its enemies, and lacked the power of 

 enrolment. Isotelus, a slower moving, but heavier-shelled 

 animal, protected itself by complete enrolment. 



^ ^-3 1^ Fig. 3. Diagram of the trails on a slab of 



^ 



^ ^^^.^ "^ sandstone found at Deschenes. 1 is the trail shown 



^ '^^^^Ci> ^ in the photograph on the plate, and 1 and 3 



^\ ^Ss ^^ are supposed to have been made by an Isotelus. 





3 



Certain trails found by the writer on the surface of slabs 

 of sandstone from the Chazy at Deschenes tend to confirm the 

 suspicion that the gnathobases were used as ambulatory organs. 

 A diagram and photograph of one of these trails is here repro- 

 duced, and it will be seen that it is exactly the sort of marking 

 that would theoretically be produced by a tiilobite which was 

 crawhng with the aid of the gnathobases only. The trail num- 

 bered I in the diagram can be traced for about 100 mm. on the 

 slab, and consists of a series of pairs of approximately parallel 

 ridges, arranged on opposite sides of a narrow furrow. The 

 ridges are incHned at angles of from 30 to 60 degrees to the 

 direction of the furrow. Each ridge is about 10 mm. long, and 

 the furrow is from 3 to 5 mm. wide. The slab is a mould of the 



