Maryland Geological Survey 38!) 



ings are shallow. The shell must have been very thick between the volu- 

 tions, and may have much contracted the opening of the umbilicus. There 

 were twelve lobes and thirteen saddles on the oldest part of the volution. 

 The fiat siphonal saddle has a minute saddle in the center and a couple of 

 inflections or marginal lobes on either side of this, and then at the ends 

 two small round saddles. The ventral lobe is very broad and the two arms 

 also broad and obscurely trilobate, each lobe being subdivided by a minute 

 saddle. The first, second and third lobes are broad at top and have an 

 unequal number of small short branches, as if they were derived from the 

 trifid type. They are all probably, however, derived from a bifid type, 

 unless exception may be made for the branches of the ventral lobe. 



" The remaining lobes have one large median saddle and an equal num- 

 ber of small lobes as if derived from the bifid type. There is a series from 

 a primitive bifid lobe, the eleventh, and only the twelfth lobe is single. 

 On the right side the twelfth lobe is on the line of involution, whereas on 

 the left side that line is occupied by a saddle. The lobes are very short 

 and broad. 



" The first six saddles have broad phylliform bases and the first five are 

 bifid on both sides, being equally divided by a small median lobe, the 

 sixth is transitional and entire ; the remaining saddles are of the same 

 type, but so short and broad that they appear to be flattened at the base, 

 and in fact are approximations to that type." — Hyatt, 1903. 



Type Locally. — Xoxubee County, Mississippi. 



Sphenodiscus lobuius is well characterized by the much compressed len- 

 ticular outline. The shell substance is very thin, showing exquisite irrides- 

 eent colors and is striated with faint and evenly spaced incrementals. All 

 of the individuals upon which the sutures can be traced are referable to 

 the race which Hyatt proposed to isolate under the name Beecheri, char- 

 acterized by a slightly higher complexity of the suture line. The differ- 

 ences are no greater, however, than those exhibited by other species, and 

 as they have, apparently, neither geographic nor stratigraphic significance, 

 there seems to be no reason for recognizing them. 



The species occurs quite abundantly throughout Prince George's 

 County, but it is exceedingly difficult to remove the soft and crumbly 



