„ ^ ^. , , BRYOZOA. 157 



Pachydictya.] 



well preserved examples a thin peristome is clearly distinguishable, and, running 

 lengthwise between the rows of apertures, a thin ridge raised considerably or only 

 slightly above the level of the peristomas. The interspaces are always' as wide as 

 the zocecial apertures, and in many specimens nearly twice as wide. One specimen 

 preserves a few " closures " or zocecial covers. These are faintly convex, with a small 

 rounded opening in the anterior half. 



Of internal characters, 1 shall mention, (1) the absence of interstitial vesicles 

 between the primitive or prostrate cells of the zocecia ; (2) the contact of those por- 

 tions of the zooecia with each other on all sides, resulting from the absence of the 

 vesicles ; (3) the peculiar convex shape of the anterior or transverse partitions of the 

 primitive cells ; (4) the density and early beginning of the solid interstitial filling, 

 and consequent indistinctness of the vesicles. Diaphragms are usually present, one 

 or two in each tube. 



Compared with other species, P. elegans is found to differ, externally, in its usu- 

 ally wider and more rapidly branching zoarium, and flatter interspaces ; internally 

 in the sljape of the primitive cell and the earlier development of the intersititial 

 vesicles. P. occidentalis is sufficiently distinguished by its mode of growth and its 

 maculose surface, and P. fimbriata by its peculiar ruffled non-poriferous margin. 



Formation and locality. — This species is one of the commonest fossils of the Galena shales, having 

 been found at perhaps every one o£ the numerous localities in the state where that horizon is exposed. 

 Also at Decorah, Iowa. It occurs also in the lower layers of the overlying limestones, at Fountain, and 

 several specimens have been collected from the Phylloporina corticosa horizon. Its wide geographical dis- 

 tribution outside of the state has been mentioned already. 



Mus. Beg. Nos. 7607, 7609, 7616, 7619, 7623, 7632, 7639, 7613, 8027. 



Pachydiotxa pumila Ulrich. 



PLATE X, FIGS. 1-4. 



Pachydictya pumila Ulkich. 1890. Jour. Cin. Soc. Nat. Hist., vol. xii, p. 186, flg. 11. 

 Bhinidictya humilis Ulrich, 189a. Jour. Cin. Soc. Nat. Hist., vol. xii, p. 185, flg. 10. 



Zoarium bifoliate, small, usually less than 1 cm. in height, growing rather 

 irregularly. Branches from 1.0 to 1.5 mm., wide, generally bifurcating at intervals 

 of from 2 to 4 mm., but some fragments observed are undivided for a distance of 6 

 or 8 mm. Zocecia in from three to six ranges, with four or five the average number 

 shortly after bifurcations. The arrangement of their oval apertures is inclined to 

 be rather irregular, though more or less longitudinal rows prevail in most cases. 

 Over the basal parts of the zoarium this irregularity is apparent in a higher degree 

 than in the distal portions. In the latter five to seven occur in 2 mm. longitu- 

 dinally. The size of the apertures and the general appearance of the surface varies 

 with age and other conditions. Nearly complete examples may show all the phases. 



