414 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
pleurocyst filling up the interareolar costules. The aperture is very oblique, invis- 
ible, hidden by the mucro; the peristome is little salient, elongated, elliptical; it 
bears proximally a small mucro triangular and salient; it bears laterally and 
distally 6 to 8 spines. The hyperstomial ovicell is imbedded in the distal zooecia; 
it is globular, salient, orbicular,' of the same nature as the frontal. The avicularia 
Fig. 121. — Genus Exochella Jullien, 18S8. 
A-G. Exochella longirostris Jullien, 1888. A. Ovicelled zooecia, X 20. B. Young un- 
ovicelled zooecia, X 30. C. Young zooecia, X 30, provided with marginal articulated spines, 
3 or 4 in number. In this singular species the peristome in growing has an abnormal tendency 
to bear either to the right or to the left, abandoning some of the marginal spines of the orifice. 
The latter are then isolated outside of the orifice, close to the line of separation of the zooecia. 
This curious occurrence is observed chiefly on the zooecia bearing only a single avicularium. 
The appearance of the avicularium seems, therefore, to be the cause of the change of direction 
of the peristomes of such zooecia. D. Orifice bearing two spines, X 100. E. Avicularia, X 
100, one of which is provided with the retractor muscle. (After Jullien, 1888.) F. Interior, 
showing the aperture and the locella, X 75. G. Opercula, X 100. (After Levinsen, 1909.) 
H, I. Exochella lobata Levinsen, 1909. H. Interior, showing the apertura, locella, and the 
vestibular arch, X 75. I. Operculum, X 140. (After Levinsen, 1909.) 
J, K. Exochella tricuspis Hindis, 1881. J. Interior, X 75, showing the vestibular arch, the 
apertura, the locella, and three coalesced teeth. K. Operculum, X 140. (After Levinsen, 1909.) 
are straight, very long triangular, with pivot, placed symmetrically and longi- 
tudinally. 
Measurements— External aperture lpi= 0.14 mm. 
Zooecia 
[ t s= o.oo mm. 
173=0.35 mm. 
Variations . — The exterior aspect is quite variable, as in all the species with 
a pleurocyst; this is not deposited on the young zooecia. The interareolar costules 
are formed first (fig. 1) ; their intervals become filled up and the zooecia are then 
covered by this finely granulated deposit which is characteristic of the pleurocyst. 
