260 



MINERALS AND GEOLOGY 



Formations. 



Examples are often found in a " rolled up " condition. 

 The genus Homalonotus has also thirteen body-seg- 

 ments, and facial suture terminating at the posterior 

 corners of the head-shield as in Calymene. But the 

 glabella is unlobed and feebly pronounced, and the 

 axis or central part of the thorax is scarcely defined 

 the longitudinal furrows, by which in most trilo- 

 bites the axis is marked off from the pleurae, being 

 in this genus very indistinct. The Homalonoti 

 are typically Devonian and Upper Silurian forms, 

 but some occur in Lower Silurian strata. Figure 

 179 represents H. delphinocephalus (usually when 

 perfect, about three inches in length) from the Niagara formation. 



5. Merostomata : The crustaceans of this Order comprise a single 

 living genus, Limulus, and several extinct genera 

 of related character in which the basal joints of 

 some of the feet fulfil the office of jaws in the pro- 

 cess of mastication. Hence the name of the group, 

 from {J.r t poq a thigh and ffr6fj.a mouth. The Meros- 

 tomes may be arranged under three families: Limu- 

 lidce, Belinuridce and Eurypteridce. 



The Limulidce are represented by the single 

 genus, Limulas, species of which are popularly 

 known as " Horseshoe crabs " or " King Crabs." 

 They consist of three essential parts : a large cres- 

 cent-shaped cephalic portion covered by a single 

 shield, a thoracic portion covered by another shield, 

 and an abdominal portion in the form of a long spine or " telson," 

 the whole presenting a trilobitic aspect. In the young state, the tel- 

 son is absent, and the trilobitic aspect is especially marked. The 

 genus dates from the Triassic period, but fossil examples are unknown 

 as regards Canadian strata. 



The Belinuridce much resemble the Limulidce in general characters, 

 but the thorax and abdomen in most forms are distinctly segmented. 

 Hemiaspis from Silurian strata, and the Carboniferous types Belinu- 

 rus and Presttvichia, are the principal genera. Examples have been 

 discovered in the United States but none in Central Canada. The 

 dorsal aspect in these forms is more or less distinctly tri-lobed. 



FIQ 179 

 Homaionotm deiphinu- 



cephalus : Green. Nia- 



fr ara Formation. 



