CRUSTACEA. 23 



on the posterior face, continuous with the test, and formed by the deep 

 deflection of the anterior margin of each segmenl and its reflection to the 

 articulating ring. They originate just within the mar-ins of the axis at 

 points which are marked upon its dorsal surface by a slight posterior deflec- 

 tion of the segments, and as they pass downward converge toward the axial 

 line where members of the same pair would nearly meet. Thirteen pairs of 

 these processes are discernible, one for the third lobe of the glabella, one for 

 the occipital annulation, and eleven for the segments of tin- thorax, the first 

 two lobes of the glabella and the annulations of the pygidium being unrepre- 

 sented. Similar processes have been observed in Phacops latifrons by von 

 Koenen (Neues Jahrb fur Min., etc., 1880, vol. i. p. 43U), and are regarded 

 by him as the liases of support for the natatory appendages. The interpreta- 

 tion of the ventral anatomy of the trilobites as given by Waleott for certain 

 species {Cahjmene senaria, Ceraurus pleurcxanihemus), would regard such pro- 

 cesses as enclosed within the "ventral membrane," and consider them 

 functionally as supports for the axial viscera. Sections of other specimens of 

 Phacops rana show r evidence of the existence of such a ventral membrane, to 

 the lower surface of which the natatory appendages were probably attached. 

 The presence of this membrane does not, however, preclude a direct muscu- 

 lar connection across the visceral cavity, between the axial processes and 

 the leer-bases. 



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Development. The members of this specie- assume very early in their life- 

 history the essential features of maturity. Notwithstanding the abundant 

 material in the collections of the State Museum, which afford gnat variations 

 in size, no noteworthy deviation in form, proportions or details has been 

 observed in the different stages of growth, except in the number of corneal 

 lenses, as noticed above. 



Dimensions. An average of several hundred measurements gives for the 

 average individual a length of 32 mm., and a width of 19 mm. Tin- largest 

 entire individual observed measures as follows : 



