NORTH AMERICAN LATER TERTIARY AND QUATERNARY BRYOZOA. 
45 
This species is closely related to A. flemingi Busk, 1854, and A. trifolium Wood, both living in the 
seas of to-day and known also as fossils from late Tertiary beds of England and Italy. It is distinguished 
from both by the lesser development of the frontal lamina. The constriction of the aperture is usually 
more pronounced in A. constricta. 
Measurements .—Opesia 
ho = 0.30 mm. 
lo = 0.22 mm. 
Zooecia 
|Zz = 0.52-0.54 mm. 
j lz = 0.36 mm. 
Variations .—The cryptocyst is very little developed; it is more like a proximal 
enlargement of the mural rim. By abrasion of the surface, the five dietellae are 
easily visible. The beak of the avicularium is salient and rounded. The two 
salient condyles arranged symmetrically in the opesium are intended to support the 
articulation of the operculum. The ancestrula is orbicular, without spines. It 
engenders one large and three small zooecia; the two groups are separated by two 
large zooecia derived from the small ones. We have observed some calcified zooecia 
perforated by a small median pore. Regenerated zooecia are rare. 
Affinities .—In a recent letter, Mr. Waters remarks that the differences between 
this species and Amphiblestrum flemingi Busk, 1854, are not sufficient to separate 
the two species. It is evident that we have here a difficult problem in specific deter¬ 
mination and that we are not yet able to distinguish the true characters of the species 
from those of a variety. The problem is further complicated by the great zooecial 
variations. 
The micrometric measurements taken on the recent specimens and on those 
from the Helvetian of Touraine and the Tortonian of Eisenstadt, Hungary, are 
always smaller than those of A. constrictum, a character which may be sufficient to 
maintain the American species. The American specimens also appear deprived of 
oral spines. However, the specific name is a small matter if the illustrations are 
good for careful comparisons. 
Occurrence .—Miocene (St. Mary’s formation): Cove Point, Maryland (rare). 
Miocene (Yorktown formation): 3 miles southwest of Petersburg, Virginia (rare). 
Pliocene (Waccamaw marl): Waccamaw River, Horry County, South Carolina 
(rare). 
Plesiotypes. —Cat. Nos. 68456, 68459, U.S.N.M. 
AMPHIBLESTRUM TENUIPARIETIS, new species. 
Plate 13, fig. 7. 
Description .—The zoarium incrusts shells. The zooecia are distinct, sepa¬ 
rated by a deep furrow, hexagonal, rounded above; the mural rim is very thin, 
rounded, very finely granulated; the cryptocyst is of little depth, concave, gran¬ 
ular; the opesium is oval, narrowed laterally by two condyles serving for articu¬ 
lation of the opercular valve; it bears a concave or undulated proximal border. 
The avicularium is elliptical, salient, with pointed beak directed below. 
Measurements .—Opesia 
ho = 0.30 mm. 
lo = 0A0 mm. 
Zooecial 
\Lz = 0.55 mm. 
[ lz = 0Ab mm. 
Affinities .—The ancestrula is orbicular, garnished with very small spines, it 
engenders three small and two large zooecia. 
This species is quite close to Amphiblestrum Jlemingi Busk, 1854; it differs 
from it in the absence of gymnocyst, and in the larger ancestrular zooecia. It 
