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NEW YORK STATE MUSEUM 



parts of the long, broad, flat lateral sides. They express for this reason, 

 quite distinctly an intrinsic tendency to a repetition of the lateral apertural 

 spines. 



Another group of spines in our material is represented by those 

 observed in Lasiograptus mucronatus, bimucronatus and 

 whitfieldi. In these the apertural spines are primarily the bearers of 

 extrathecal structures, consisting of a network of fibers etc., but they indi- 

 cate by their rigid stout form that secondarily they also acquired a defensive 

 function [see plate 30]. In Memoir 7 (p. 518, 539) it has been claimed by 

 the writer that the extensive perforations and the final reticulation of the 

 peridermal walls in the Retioloidea are expressions of a strong tendency to 

 lighten the rhabdosome, resulting from the adoption of a planktonic mode 

 of life. In Lasiograptus [see postea] the extrathecal fibrous structures are 

 either the result of progressive differentiation of former spines, as generally 

 assumed, or, as suggested by Tornquist [Silj. grapt. I, t. 2, p. 2 7] the distal 

 parts of thecae that have been dissolved into meshes. If the latter inference 

 \s* \ is correct, the spines of Lasiograptus are not homo- 



\^*~ ^rx logous to any of those of the preceding groups. In 



that case they result from what Beecher has termed 

 " restraint of environment causing suppression of struc- 

 tures" The environment, i. e. the open sea, was not 

 favorable to the development of strong peridermal 

 walls and led to a gradual suppression of this structure, 

 the suppression in this case beginning at the apertural 

 end of the theca and proceeding in proximal direction, 

 leaving the spines as principal vestiges of the process. 

 Entirely different again in origin and character is 

 the spinose appearance of certain species of Mono- 

 graptus and Rastrites [see text fig. 10, 1 1 |, which is especially well expressed 

 in several of Perner's accurate drawings [see his pi. 13, fig. 2y, 2$, 29 ; pi. 1 2, 

 fig- T 7> 2 5» 3°'. l'l- 37 cl a ^\ * n tncsc th( ' spines are not integumental 

 excrescences but the thecae themselves assume the form of spines, the aper- 



Fi^. 10 Rastrites lin- 

 naei Barrande. Copy from 

 Perner. x 4 



Fig. 11 Monograptussp. 

 Copy from Perner. x 4 



