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 paleeozoic seas. As in Limulus 
ibs 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, Darrande, and others, that 
it has been thought to be both an inhabitant of shallow waters along the 
coasts and also of the deeper seas. Itis found in both littoral and deep- 
water 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, à 
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 F'orm and Number of the Appendages in Vari- 
ous Genera and Species. — “We have presented to us in the Crustacea 
probably the best zodlogical 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 
* Seo Dr. Packard's description of the spawning of Limulus and its probable 
occurrence in the same manner with the Trilobite. Ibid., p. 186. 
