76 NEW YORK STATE MUSEUM 



growth-stages are very apt to pass through a stage with farther for- 

 ward-carried genal spines than are found in the mature stage (see 

 text-figures, 15-17 and Walcott, 1886, pi. 20). And as in the case of 

 Olenellus gilberti and O. fremonti closely related 

 species with normal genal spines (O. gilberti) occur associated 

 with those with laterally placed genal spines (O. fremonti) to 

 such an extent that Walcott formerly (1886, p. 21) considered the 

 latter as abnormal forms of the former ; and the species fremonti 

 possesses not only in the forward position of the genal spines but also 

 in the greater expansion of the anterior glabella characters that 

 suggest its retarded development as compared with O . gilberti. 

 It might thus fall under the form of " reversion " cited by Grabau. 

 It is likewise possible that the forward carrying of the genal 

 spine in the ontogeny was inherited in those species of Triarthrus 

 that possessed genal spines, from the Cambrian ancestors and 

 retained even as long as late Ordovician time in the ontogeny. 

 The apparent reversion in T. spinosus would then, as far as 

 the genal spines are concerned, be a further development of an 

 ontogenetic feature retained from the Cambrian ancestors. Pos- 

 sessing no growth-stages of either T. spinosus or earlier 

 Triarthri with genal spines no definite conclusion can be reached as 

 to this character. 



It is different in regard to the series of long axial spines of the 

 posterior thorax. In this case one might feel tempted to infer that 

 meristic variation in form of increase has taken place pointing to 

 the axial spine that is carried on the occipital ring in T. spinosus 

 as an inheritance of this almost ever-present organ in the Cambrian 

 trilobites. A comparison of Triarthrus spinosus with T. 

 e a t o n i and b e c k i , that possess a continuous series of mucros 

 on the axis of the thorax, on the one hand, and with the associated 

 T . g 1 a'b e r , that is completely smooth, leaves little doubt that 

 in the axial spines of T. spinosus we have to do with organs 

 that were never actually lost, as shown by the series of axial mucros 

 of T . b e c k i and e a t o n i , but had become more or less sub- 

 merged (entirely in T . glaber) through the influence of inhibit- 

 ing factors. 



It is a peculiar fact that the axial line of spines, is so prominent 

 in the Cambrian trilobites that the genera Albertella, Zacan- 

 thoides, Neolenus, Elliptocephala, Mesonacis, 

 C a 11 a v i a , H'o Imia, Wanneri a, Paedeumias, 

 Bathyuriscus, Dolichometopus, Ogygogopsis 



