448 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
are elongated the more the peristomice is enlarged and the more the micrometric 
measurements are increased. These are, therefore, often inferior to those which 
we give above; it is not rare to observe; Zs=0.80-1.00 by fe=0.40-0.45 mm. (fig. 8) 
Very rarely a thin separating thread limits the zooecia (fig. 6). 
One of the distinctive characters of this species is a sort of small lyrula 
which is visible on the proximal border of the apertnra in the interior of the 
zooecia (fig. 13). 
The specific character of the species is the presence of the frontal sulci at the 
base of which the tubules open ; they give to the tangential sections an especial and 
strange aspect (fig 15). 
Affinities .- — The species differs from Metradolium obliquum in which the 
rimule-spiramen is identical, in its frontal sulci and in its larger dimensions 
(Zs>0. 80 mm) . 
It differs from Metradolium transversum also provided with sulci in its very 
large and unsymmetrical rimule-spiramen and in the almost constant presence of 
two oral avicularia. 
O ocurrence . — Claibornian (Gosport sand) : One mile southwest of Rockville, 
Clarke County, Alabama (rare). 
Lower Jacksonian (Moodys marl) : Jackson, Mississippi (very common). 
Cotypes. — Cat. No. 63850, U.S.N.M. 
ANALYTICAL KEY TO SPECIES OF METRADOLIUM. 
| Spirainen present , 
[No spiramen 
Large, salient lip between the spiramen and the peristomice 
No lip to the peristomice 
f Small, little distinct rimules 
[Large, very distinct rimules , 
Large, transverse peristomice, often with sulci 
Small peristomice (7pe=0.10-0.12 mm.) _ 
Large, symmetrical rimule, straight, rounded 
Large, unsymmetrical rimule, one lip being much lower than the other 
I Large tremopores, large micrometri'c dimensions 
Small tremopores, oblique rimule 
Tremopores at the base of the sulci 
Genus LEIOSELLA Canu and Bassler, 1917. 
1917. Leiosella Canu and Bassler, Synopsis American Early Tertiary Cheilostome 
Bryozoa, Bulletin 96, United States National Museum, p. 48. 
The frontal is an olocyst. The peristomice of the ovicelled zooecia is of different 
form from that of the other zooecia ; it is a lunar crescent and deprived of rimule- 
spiramen. 
Genotype. — Leiosella rostrifera Canu and Bassler, 1917. Vieksburgian. 
This genus differs from Metradolium only in the nature of the frontal which 
is here a very thick olocyst. 
Of the three species mentioned, one only bears o vice] Is. Further researches 
are necessary. 
2. 
3. 
M. labratulum. 
M. dis simile. 
4. 
5. 
M. transversum. 
M. parvirimulatum. 
M. conveniens. 
6 . 
M. grande. 
M. obliquum. 
M. sulciferum. 
