REPORT OF THE DIRECTOR I919 'JJ 



and Asaphiscus contain species with rows of long axial spines 

 or prominent mucros; while in the following Ordovician to 

 Devonian periods this feature becomes suppressed to such an extent 

 that, for example, in Barrande's wonderful array of Ordovician, 

 Silurian and Devonian trilobites not a single form with this row 

 of spines is found,^ while Hall and Clarke (1888) found among the 

 Devonian trilobites of New York but a single Dalmanites 

 (Cryphaeus, namely, D. (Cryphaeus) boothi var. 

 c a 1 1 i t e 1 e s , one Phacops (P. cristata) and one C a 1 y m- 

 mene (C. platys) retaining faint tubercles on the axial line, 

 and none, not even the highly spinose Acidaspidae exhibit rows of 

 axial spines. 



Now and then, however, a form will develop a single spine on one 

 of the thoracic segments or the axis of the pygidium, as L i c h a s 

 (Conolichas) eriopis Hall on the pygidium, and the strange 

 Cyp'haspis ceratophthalmus Goldfuss (see Richter, 

 1918, pi. 2, fig. 13) which has a single spine on the middle of the 

 back. It is legitimate to consider these also not as new creations or 

 reiterative developments, but as a local awakening of the dormant 

 or latent axial spines 01 the Cambrian ancestors, reduced in the few 

 Devonian forms cited above to the rudimentary vestiges represented 

 by the small mucros. All these features, the axial series of spines 

 or the single isolated spines, and the axial rows of mucros are thus 

 the last traces of a character highly developed in the Cambrian 

 ancestors and when thus appearing abruptly, as in T. spinosus 

 or Cyphaspis ceratophthalmus partake of the character 

 of atavisms or reversions, as popularly understood but in reality are 

 revivals of latent characters that were never lost, and quite surely 

 not cases of reiterative development. 



The cause of this remarkable resuscitation of the axial spines, 

 evidently once so useful to the Mesonacidae and other Cambrian 

 trilobites, may well be sought in an adaptation or return to similar 

 habits. Rud. Richter has lately shown in an interesting series of 

 papers on the structure and life habits of trilobites, that many of 

 the spinose appendages do not only serve for protection, or are 

 merely spinose excrescences indicating gerontic condition as Beecher 

 has urged, but had a very important function in allowing the crea- 

 tures to float with little exertion in the water through the great 



" Among the Proetidae Astycoryphe gracilis Barrande, however, 

 is figured and described with a row of mucros by Richter (1919, p. 12 & 13) 

 and A. senckenbergiana R. and E. Richter with mucros on the last 

 four thoracic segments. 



