522 Systematic Paleontology 



Length of left valve .84 mm. ; height .40 mm. ; length of spine .97 mm 

 Occurrence. — Hklderberg Formation, New Scotland Member. 

 21st Bridge. 



Collections.— Mdixy\a'n.(i Geological Survey, U. S. National Museum. 



Genus MESOMPHALUS n. gen. 



Carapace of medium size, 1 mm. to 21 mm. in length, equivalved ; valves 

 moderately convex, oblong subquadrate in outline, with a small central pit 

 lying between two rather small rounded nodes ; edge of valves rather thick, 

 usually concealed by a flattened border which overhangs it; ventral pouch 

 somewhat elongate, sausage-shaped, occupying nearly the full length of the 

 ventral side. 



Type Mesomphalus Jiartlei/i n. sp. 



This new genus is founded upon an ostracod occurring abundantly in 

 the Keyser limestone of Maryland and West Virginia, and exhibiting 

 features which relate it on the one hand to Beyrichia, and on the other to 

 Primitia, with a suggestion also of Ulrichia and Hollina. The presence 

 of a large ventral pouch recalls this persistent feature of Beyrichia, but 

 aside from this character, there is really very little in common between 

 the two genera. The characteristic three lobes of Beyrichia are totally 

 wanting. The true affinities of the genus lie with the Primitiidse, as 

 indicated by the distinct subcentral pit, while the two nodes are suggestive 

 of Ulrichia. From these two genera it is clearly distinguished by the 

 presence of the ventral pouch, and in individuals not possessing this, by 

 the subcentral instead of' nearly dorsal location of both the pit and node. 

 The Carboniferous species of Hollina, like R. radiata (Jones), also have 

 two somewhat similar rounded nodes, but if they are really homologous 

 with those on the valves of Mesomphalus, an obvious difference is apparent 

 in the location of the lobes in that genus, only one of them being situated 

 in the middle part of the valve, the other, the anterior, being far up near 

 the dorsal edge. 



The closely approximated nodes, their subcentral position, the central 

 pit, and the long, sausage-shaped ventral pouch, are the distinguishing 

 characteristics of this new genus. 



