254 • Field Columbian Museum — Geology, Vol. III. 



union with the pubic process. From the femoral notch to the median 

 line the margin is elevated to form a sharp ridge, deeply pitted on 

 the free surface. 



The dermal shields of the plastron are very complex in arrange- 

 ment. The intergulars are similar on the inferior surface to those 

 of B. variolosus , but superiorly they are extended to cover the pro- 

 truding gular process. Between them the median sulcus twice crosses 

 the median suture. The gulars are long and narrow, and meet at the 

 median line. The humerals are relatively smaller than in B. vario- 

 losus. They present for union at the median line a surface barely 

 a half -inch in length. The pectoral shields as a pair are similar in 

 outline to those of the species cited, though they extend somewhat 

 farther backward mesially. Between them the median sulcus de- 

 parts from the median line in a series of meanderings in which it 

 crosses the median suture five times. In the abdominals it follows 

 almost parallel with the suture making a single digression near the 

 posterior border. The abdominals send backward a pair of peculiar 

 narrow processes between the inguinals and femorals to the border 

 of the femoral notch. The femorals are sub-quadrangular in outline, 

 extending laterally to the border of the femoral notch. In the anals 

 the median sulcus presents the most fantastic tracery, crossing the 

 median suture in a series of wide loops, departing from it, in one in- 

 stance, nearly two inches and finally returning to it at the posterior 

 margin. On account of portions missing at the lateral angle of 

 the carapace it cannot positively be determined whether the 

 marginals are divided into an inframarginal series. From the fact 

 that no evidence of a dividing sulcus can be made out on either side, 

 as well as the opposite position of the vertical sulci on each side of 

 the gap, it is assumed that the series is not divided into marginals and 

 inframarginals, respectively. 



From the narrow lateral extremities of the pectorals radiate 

 three wedge-shaped shields. The anterior and smallest is the axillary 

 shield which overlies the axial buttress and extends upward along 

 the free margin of the carapace to join the second nuchal. The 

 other shields radiating from this point are the fourth and fifth 

 marginals. The sixth marginal is very broad at its ventral 

 border, joining nearly the whole lateral surface of the abdominal 

 shield. Two pairs of shields converge at the postero-lateral 

 angle of the abdominals. The most anterior is the seventh mar- 

 ginal, which narrows mesially almost to a point. Back of this lies 



♦Proc. Acad. Nat. Sci. Phila. 1876, p. 257. 



