MUSEUM OF COMPARATIVE ZOOLOGY. 215 



between the young Limulus and the young Trilobites as described by 

 M. Barrande, there is no reason to doubt that the young Trilobite 

 may have had the same power of distributing itself and its species 

 over extended areas in the wide-spread palaeozoic seas. As in Limulus 

 its later growth changed its manner of life, and its movements were 

 finally restricted to crawling about the sea bottom in search of food. 

 We have seen from the views of Burmeister, Barrande, and others, that 

 it has been thought to be both an inhabitant of shallow waters along the 

 coasts and also of the deeper seas. It is found in both littoral and deep- 

 Avater formations. Muddy or sandy, fine or coarse, hard or soft, argilla- 

 ceous or calcareous deposits, it occurs in all. With these facts in view, 

 it is probable that it ranged along the shore in quiet bays, and also in 

 the habitat of the Brachiopods and other deep-water Invertebrates. In 

 conclusion we may say that the Trilobite in its younger stages of growth 

 was active and a free swimmer, thus distributing itself over broad areas. 

 That on reaching a larger growth it became more limited in its 

 natatory powers and crawled about the bottom in search of soft-bodied 

 organisms for food and during the spawning season for a place to de- 

 posit its eggs.* From the presence of the swimming joints on the 

 posterior pair of cephalic appendages it may have had limited natatory 

 powers during the latter part of its existence. 



Of the power to enroll itself and thus protect its vulnerable ventral 

 surface from attack by an impenetrable coat of armor, the sections cut 

 of Calymene and illustrated on Plate I. figs. 8 and 9, which were 

 cut from an enrolled specimen, and Plate III. figs. 1 and 2, from one 

 partially coiled, abundantly prove. 



In the '• Geology of Canada," p. 104, a number of tracks or trails of 

 Crustaceans are illustrated. They occur in the Potsdam sandstone, a 

 formation with an abundant Trilobitic fauna. From the structure and 

 form of the legs of the Trilobites it is very probable that these tracks 

 (Protichnites) were made by them. 



Variation of the Form and Number of the Appendages in Vari- 

 ous Genera and Species. — ''We have presented to us in the Crustacea 

 probably the best zoological illustration of a class, constructed on a com- 

 mon type, retaining its general characteristics but capable of endless 

 modifications of its parts, so as to suit the extreme requirements of 

 every separate species." (Woodward.) When the great extent and 

 variety of the modifications of the dorsal shell of the Trilobite are taken 



* See Dr. Packard's description of the spawning of Limulus and its probable 

 occurrence in the same manner with the Trilobite. Ibid., p. 186. 



