526 CRUSTACEA. 



rows of Trilobites, and that it can be shown to be sometimes connected 

 with footprints consisting of a double series of transverse markings. In 

 form, Rusichnites (fig. 381, D) presents itself as an oval, cylindroidal 

 body, deeply furrowed, or bilobed, by means of a longitudinal sulcus, 

 the lateral halves being transversely ridged or grooved. The body 

 thus constituted may be independent, or may stand in apparent con- 

 nection with a cylindrical and slender appendage. 



With regard to their systematic position, the Trilobites have been 

 usually regarded as Entomostracous Crustaceans, with relationships 

 to the Phyllopods and to the Merostomata ; but they have been 

 sometimes considered as related to the Isopods. With regard to 

 these last, there is no doubt a considerable general likeness between 

 such an Isopod as Serolis and various forms of the Trilobites \ and 

 a tangible point of resemblance is found in the fact that in many 

 Isopods the caudal segments coalesce to form a shield like the 

 pygidium of the Trilobites. On the other hand, the segmentation 

 of the body in the Trilobites is indefinite, the number of limbs is 

 large, and thoracic branchiae are developed ; whereas in the Isopods 

 the segmentation of the body is definite, there are only seven pairs 

 of legs, and the branchiae are abdominal. With the Phyllopods, 

 the Trilobites are in agreement as regards the indefinite segmenta- 

 tion of the body ; and the recent genus Apus possesses a lip-plate 

 which in form is very similar to that of the Trilobites. The Trilo- 

 bites, however, have a calcareous crust ; 

 their limbs are jointed, and not phyllo- 

 podous ; and the segments of the ab- 

 domen are anchylosed. There are, 

 finally, remarkable points of relationship 

 between the Trilobites and the Mero- 

 stomata, and particularly between the 

 former and the Limuloid types of the 

 latter. Thus, the Trilobites show affin- 

 ities with the Merostomata in the pos- 

 session of a very similar head-shield to 

 that of the latter, the presence (particu- 

 Fig. 382.— Larva of Limuius on larly in some extinct Limuloids) of dis- 



hatching, greatly enlarged. (After ^ indications of & « fedal sutme » the 



existence of compound sessile eyes, and 

 the microscopic structure of the crust. Moreover, the larva of the 

 recent Limuius (fig. 382) is destitute of the tail-spine of the adult, 

 and in many respects shows a striking resemblance to certain of the 

 Trilobites, and particularly to the genus Trinucleus. There are, 

 however, various striking points of difference between the Trilo- 

 bites and the Limuloids. Thus the former always possess free 

 thoracic segments, and the appendages carried on these differ en- 



