89 



or neck-segment, and the two curved ones attached to the posterior 

 angles of the cephalic shield, give as many as six spines, which Triar- 

 th'tis spinosus, Billings, possessed. These facts are not founded on this 

 specimen alone, for there are several others, less perfect however, which 

 show the same characters. Of those three spines on the back of the 

 trilobite we find that the anterior one appears to be the shortest ; it, 

 however, extends " beyond the apex of the pygidium ;" the posterior 

 one is somewhat longer than the anterior, whilst the intermediate one 

 is vei-y long indeed — about twenty millimetres ; it is the longest spine 

 of the individual, and projects over ten millimetres beyond the apex of 

 the pygidium. All these spines are more or less cylindrical, attenu- 

 ated to a sharp point— the longest one appears to be grooved below, no 

 doubt to allow it to rest partially over the posterior one. (See Plate.) 



It is somewhat difficult to imagine how it happens that so many 

 specimens have been collected by difiei'ent individuals, which show only 

 one spine on the thoracic segments and that on the eighth. It maybe due 

 in part to the state of preservation of tlie specimens and delicacy of these 

 spines, which, perhaps, were quite brittle (a mere fragment of the point 

 of attachment ofuen indicating the previous presence of the spine), else 

 some solution may be found in the fact that the eight anterior thoracic 

 segments are lield so close together, being scarcely ever separated, whilst, 

 beginning with the ninth thoracic segment, the remainder of the seg- 

 ments, down to the pygidium, are almost invariably found detached and 

 separated from each other. 



T. spinosus, when adult, has been observed to possess as many as 

 thirteen segments in the thorax (Billings and others) ; therefore, between 

 the eighth thoracic segment and the pygidium, there would be normally 

 five segments ; of these, we know that two at least possessed spines. 

 From the evidence gathered in the examination of numerous specimens, 

 it would not be at all surprising to discover that from the eightli to the 

 thirteenth segments each had a spine proceeding fiom the centre of the 

 axis ; were such the case the species would be much more spiny than at 

 first it was thought to be —as it would possess not " four," nor " at least 

 more than foui'," nor more than '■ six," but nine spines. 



