538 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



deeper and more complete separation of the two halves is so prevalent 

 that this seems to be the only explanation to be offered for its appear- 

 ance. Their absence in the few specimens is probably due to defect of 

 preservation. The common albuminous floor and the upper and outer 

 albuminous coating are of remarkable thickness, varying from one to 

 three millimeters ; but the coating attenuates to a mere lamella as it 

 passes down the median furrow, so that when the mass remained quiet 

 in the position in which it was laid, the lateral halves pressing closely 

 against each other, the combined thickness of the two albuminous 

 walls would together no more than equal the ordinary thickness of the 

 albuminous partition between any two contiguous eggs. That such a 

 partition existed even in those which do not show it now, seems probable 

 from the regularity of the furrow in every instance of its occurrence and 

 by its prevalence 5 some specimens merely show a sharp groove along 

 the middle, the halves remaining in complete juxtaposition j* others 

 again are so completely separated as to be curled over and meet beneath. 



This, together with the fact that the egg-mass is otherwise extremely 

 regular (showing only so little plasticity as to allow one broad side to 

 be straight, while the opposite is a little convex), and never exhibits 

 the slightest tendency to coil longitudinally, leads me to believe that the 

 egg-masses were laid in the water of shallow basins, upon the muddy 

 floors, which could be reached by the abdomen of the insect while rest- 

 ing upon a stone or overhanging twig. In this medium, the albuminous 

 secretion would expand to the utmost ; if the bunch of eggs remained 

 undisturbed, it would present us with the more regular hirudiform masses 

 that have been found; if rolled about by the disturbance of the waters, 

 the two halves would curl toward each other more or less closely, form- 

 ing a subcylindrical mass, and inclose between their approaching walls 

 more or less of the mud in which they are rolled. This is exactly the 

 appearance of most of them now, inclosing the same substances as that 

 within which they and the accompanying Bulimi and other fresh-water 

 mollusks lie imbedded.t 



These masses differ further from those of Corydalus in the extraordi- 

 nary amount of albuminous matter by which both the entire mass and 

 each individual egg was surrounded. This is perhaps to be explained 

 by the medium in which they appear to have been laid, and will in part 

 account for the vast size of the ootheca, which are much larger than 

 any mass of insect-eggs which I can find noticed. The size of the mass, 

 however, is also due to the greater magnitude of the eggs themselves, 

 which are twice as long and proportionally larger than those of Coryda- 

 lus, and lead to the conviction that we are to look in the rocks of the 

 earliest Tertiaries for an insect of great magnitude, closely allied to our 



* These specimens are some from which, weathering has removed their outer albu mi- 

 nous coating; perhaps, if this had remained, the furrow would have been concealed by 

 the complete union of the attingent albuminous walls. 



t The deposit in which they occur is a fresh-water one, but Mr. Lesquereux informs 

 me that brackish-water forms are found both above and below them. 



