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BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
the species is now therefore rather large, and it may become convenient perhaps to 
make some restrictions. The avicularia are quite variable. Rhagasostoma cingens , 
R. hexagonum , and R. circumvallatum Koschinsky, 1885, from the Lutetian of 
Bavaria, have falciform onychocellaria. Rhagasostoma dutempleanum D’Orbigny, 
1 852, and R. pugeti Canu, 1907, of the Parisian Lutetian, have an avicularium with 
double orifice. Rhagasostoma prominetis , Canu, 1907, also from the Parisian Lute- 
tian, possesses an avicularium with two to four false opesiules. Many Cretaceous 
species of Rhagasostoma possess the delicate avicularia called reticulocellaria by 
Canu. 
Such variations exist likewise in Aspidostoma. Aspidostoma giganteum Busk, 
1854, and A. globiferum Canu, 1911, have only small frontal or marginal avicularia. 
The other species have interzooecial avicularia. Aspidostoma poriferum Canu, 1904, 
A x 17 
Fig. 66. — Genus Rhagasostoma Koschinsky, 1885. 
A. Rhagasostoma hexagonum Koschinsky, 1885. Zooecia, X 17 (after Koschinsky, 1885). B. 
O. Rhagasostoma crassa, Beissel, 1865. B. Several zooecia and onychocellaria, X 17. o, passage 
of the occlusor muscles of the mandible; on, onychocellarium ; op, opesiule; r, passage of the 
elevator muscles of the mandible. 
C. Internal cast, X 30. (B, C after Beissel, 1865.) 
has an avicularium with lateral teeth, as lias also A. incrustans Canu, 1908. There 
is an avicularium with pivot in Aspidostoma flammulum Canu, 1908. Finally, 
Aspidostoma onychocelliferum Canu, 1911, has perhaps a real onychocellarium. 
Rhagasostoma is the Aspidostoma of the northern hemisphere. In Europe the 
genus dies out in the Lutetian. Here, in America, it survives into the Vicksburgian. 
In Australia it is found in the Miocene. 
Rhagasostoma dutempleanum D’Orbigny, 1852, and R. spiniferum Canu, 1913, 
appear to form a distinct genus on account of the nature of their ovicells. 
RHAGASOSTOMA LEVIGATUM, new species. 
Plate 39', figs. 6, 7. 
Description. — The zoarium incrusts shells. The zooecia are distinct, elongated, 
separated by a furrow; the mural rim is complete, salient, a little broader at the 
