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 



Ak 



A<20 



D 



B. Young un- 



H -75 



FIG. 121. Genus Exochella Jullien, 1888. 



A-G. Exochella, longirostris Jullien, 1888. A. Ovicelled zooecia, X 20. 

 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 looata Levinsen, 1909. H. Interior, showing the apertura, locella, and the 

 vestibular arch, X 75. I. Operculum, X 140. (After Levinsen, 1909.) 



J, K. Exochella trlcuspis Hincks, 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 IpiO.li mm. Zooecia , " ' ' mm ' 



I lz = 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. 



