78 Field Museum of Natural History — Geology, Vol. IV. 



of the occipital furrow but narrower and deeper; they terminate abruptly 

 before the genal angles are reached; a deep elongate pit on each fixed 

 cheek and a longer, shallower one on each free cheek represent the 

 lateral marginal furrows. 



Thorax not observed. 



Pygidium small, sub-triangular in outline, aside from the spines; not 

 distinctly trilobed, composed of three segments; the first of these has its 

 extremities produced into long, diverging, slightly recurved spines; 

 margin entire, with its ventral, surface forming a thick doublure. 



Surface of the globular portion of the glabella pustulose; pustules 

 rounded, larger near the transverse glabella furrow and gradually dimin- 

 ishing in size anteriorly; balance of cephalon smooth or finely granulose; 

 surface of pygidium pustulose, pustules more prominent on the spines. 



This species is the most abundant trilobite in the shales of the 

 Lower Maquoketa beds, but a great majority of the individuals are 

 represented only by the globular portion of the glabella. They range 

 in size from 2.5 mm. to 7.5 mm. in diameter. The writer was fortunate 

 enough to obtain about twenty more or less complete cephalons and 

 three pygidia. No thorax has been observed that can be referred to this 

 species so that it is not certain that these pygidia belong to the cephalons, 

 yet from their form and the conditions under which they were collected, 

 there is little doubt that they belong to this species. 



S. maquoketensis differs from all previously described species in 

 possessing large pits in place of the lateral marginal furrows. It re- 

 sembles S. granulata Angelin in the form of the marginal outline, but in 

 S. maquoketensis the cephalon is longer in proportion to the width and 

 the pustules on the glabella are finer. S. salteri Billings is from a similar 

 horizon, but in S. salteri the width of the glabella at its posterior margin 



