CRUSTACEA. 
23 
on the posterior face, continuous with the test, and formed by the deep 
deflection of the anterior margin of eacli segment and its reflection to the 
articulating ring. They originate just within the margins of the axis at 
points which are marked upon its dorsal surface by a slight posterior detlec- 
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 the 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 fitr Min., etc., 1880, vol. i, p. 430), and are regarded 
by him as the bases of support for the natatory appendages. The interpreta¬ 
tion of the ventral anatomy of the trilobites as given by Walcott for certain 
species {Calymene senaria, Ceraurus pleurexanthemus), 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 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 leg-bases. 
Development. The members of this species 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 great 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. The largest 
entire individual observed measures as follows; 
