152 TERTIARY INSECTS OF NORTH AMERICA. 



width of 0.6 mm ; they are nearly cylindrical, but faintly arcuate, slightly at- 

 tenuated at the anterior extremity, and slightly tumid on the posterior half, 

 at the tip of which they taper rapidly, rounding off to a rather broadly con- 

 vex extremity, which is flattened or often sunken in a circular central space 

 O.l mm in diameter (Fig. 7), outside of which the surface is rather profusely 

 filled with very shallow, obscure, circular pits, averaging 0.01 mm in diameter. 

 The anterior extremity (Figs. 5, 6) terminates in a slightly elevated, thin, 

 subtuberculate rim, inclosing a terminal portion, whose surface gradually 

 rises centrally to form a truncated cone, and is pitted with saucer-like depres- 

 sions, gradually diminishing in size up the sides of the central extension ; 

 the latter is about as long as the breadth of its tip ; its extremity (Fig. 18), 

 0.040.055 mm in diameter, is more or less sunken, with a central circular 

 pit (the micropyle) 0.01 mm in diameter ; while the rounded margin of the ex- 

 tension is made more or less irregular by the saucer-like depressions which 

 surmount it, but have now become of extreme minuteness. 



This structure of the anterior extremity of the egg agrees with what 

 was previously known of the egg of Sialis, but no mention of the elevated 

 point was made in Mr. Riley's description of the egg of Corydalus. It oc- 

 curs there, however, as I find by examination of eggs he has kindly sent me. 

 These eggs of Corydalus (Figs. 17, 22) also show the sunken space at the pos- 

 terior end, and the sides of the egg are marked nearly as in the fossil, the 

 surface of the latter being broken up by scarcely elevated, slight ridges into 

 obscure, transverse, hexagonal cells, one-tenth of a millimeter long (across 

 the egg) and one-fifth as broad, those of adjoining rows intercligitating. 



In the disposition of the eggs also these masses differ from those ot 

 Corydalus, for they are arranged in a radiating manner around the longi- 

 tudinal axis of the ootheca. All of them partake of this arrangement 

 even when, as rarely happens, there are two layers in place of one over 

 parts of the mass ; in no case are any of the eggs packed in irregularly, 

 as is the case with a portion of those of Corydalus, according to Rile}-. 

 As in Corydalus, however, the posterior ends are those which are directed 

 toward the upper albuminous coating, which in many cases shows very 

 slight subhexagonal or circular depressions or elevations corresponding to 

 the position of the extremity of the egg beneath, just as in Corydalus the 

 posterior ends of the eggs show "like faint dots through the white cover- 

 ing." The outer albuminous coating appears in the fossil to be made up of 



