SECONDARY AND TERTIARY FORMATIONS OF NORTH AMERICA. 143 



sloping, sometimes a little convexly, to the edge of the cellules. Mouth anterior, sub- 

 quadrate, with the sides curved outwardly. A special pore, which has sometimes 

 been taken for the true aperture, is always placed in advance of the mouth, sometimes 

 encroaching considerably on the proximal margin of the succeeding cellule. Surface 

 of the cellule, when in its normal state, pierced by three pairs of "special fossets " and 

 one odd one at the proximal end, making seven, placed at regular intervals behind 

 the mouth. In young cellules, and probably at times as the result of injury to the 

 specimen, we find the surface plate absent, and in its place only the bases of the pro- 

 cesses between the fossets, leaving the surface of the cellule open and merged into the 

 mouth. Under surface marked by irregularly undulating impressed lines, sometimes 

 dichotomous, with the surface between them generally minutely pustulate. 



From the Miocene. Appears to occur everywhere in this formation, from New 

 Jersey to South Carolina. Prof. Holmes quotes it as occurring in the Post-pliocene. 



We place Dr. Emmons' determination of D. umbellata as a synonym, believing that 

 on further examination it will be found to be the form of this species described by 

 Mr. Conrad under the name of L. depressa. The latter form is undoubtedly the same 

 ■ as denticulata, since we have not only had the privilege of studying Mr. Conrad's types 

 of both the so-called species, but have, by the assistance of a number of other speci- 

 mens, been able to trace the variations between them. 



The special pore, with the minute chamber at its base, called by Mr. Lonsdale the 

 "gemmuliferous (?) vesicle," is somewhat variable in form. Its mouth varies from 

 circular to crescentic, in which case the convexity points indiscriminately to the right 

 or left. It is generally surrounded by the boundary walls of the cellules, which is 

 merely pierced in the surface of the cellule and has no special boundary or lip. 



The special fossets piercing the surface of the cellule are always arranged in pairs, 

 generally placed opposite each other, except the odd one at the posterior or proximal 

 end, which is usually the largest, being sometimes almost an equilateral triangle. 



It is somewhat remarkable that only the imperfect forms of the cellules should 

 have been heretofore described, when in Mr. Conrad's types some perfect cellules are 

 apparent, and Mr. Lonsdale seems to have seen them also, since he says " mature 

 cells covered." 



In some cases, in old cellules the fossets and mouth are nearly or entirely oblite- 

 rated, but this is 'of rare occurrence. It is much more common to find merely the 

 denticulated margin. When the surface of the colony is much worn, so as to oblite- 

 rate the crest of the dividing walls, the cellules seem to be arranged in nearly radi- 

 ating lines, the quincuncial appearance being hidden by the much more obvious one 

 of the linear arrangement. On close examination of more perfect specimens, however, 

 we find that both modes occur simultaneously. 



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