16 AMERICAN HYDR01DS. 



VIII, fig. 8), and T. inum rx<i ; in several species of Sertularia, us X. </,.v/<//V/,.v (Plate III, tig. 2); 

 and in ticluy hwps/'s, as 8. <-i/l!n(lr!i-<i (Plate, XXXIX, tigs. 7 anil 8). 



(b) When tlie neck of the flask becomes elongated and curved to one side, we have what may 

 be called the ''linttli-x/i/iju'/r' hydrotheca, \vhich is characteristic of the genus Al>i<-1in<iri<i as 

 used in this work and well shown in such species as .1. <il>i<-tin. (Plate XXXII. fig. 1). .1. mr!<i- 

 /'///* (Plato XXXII, fig. 5), .1. ffrdcilis (Plate XXXV, tig. -2). The bottle-shaped hydrothec;e 

 intergrade with the flask-shaped hydrotheca' on the one hand and the triangular forms on the 

 other. 



(<) Piti-licr-xfiujicd hydrotheca? are found in certain species of Diphasia, as />. faUn.c (tig. 25), 

 D. I'dxnci/i (Plate XXVIII, fig. 4), I), paarmanni (Plate XXXI, tig. 5), and occasionally in 

 S, i-tiilin-i'llii. as in .V. ,'j>!x,-jmx (Plate XXVI, tig. 7). Such forms are produced by having the. 

 margin of the hydrotheca' expanded and sinuous on the abcauline side so as to resemble the lip 

 of a pitcher. 



(d) Biii-rcl-xliitpctl hydrotheca' are round, with both distal and proximal ends slightly dimin- 

 ished in diameter, and both upper and lower profiles coin-ex. They are more nearly radial in 

 symmetry than most of the others, and are found almost exclusively in the genus Sertularella, 

 as ,V. i-n</i>x,t (fig. 2(>). X. <n->'i/i (Plate XVII, tig. (), S. ij.n'n-iilntii (Plate XVI, tig. i>), X. jmt,,,,,,- 

 nica (Plate XVI, fig. 3). This form is usually provided with a square collar, which interferes with 

 the symmetry of the "barrel." In .V. t<nm<ri (Plate XVI, tig. 1), a very large and beautiful 

 hydrotheca is seen which, in some instances at least, attains almost a perfect barrel shape. 



(c) When this latter type is greatly elongated a slender terete outline is produced, which 

 gives the typical fusiform hydrotheca, which is very rare, illustrated by Sertularella fusiformis 

 (Plate XX, fig. H), and less perfectly by ,V. //ii/u/tfn/ (Plate XIX, fig. 7). 



(f) In a few cases the axis of the hydrotheca is straight and the base expanded, while the 

 diameter gradually decreases toward the distal end. Thus a conoid form is produced, which is 

 quite rare. Illustrated by Af>/'< tiimriu </!< .i-iimli-r! (Plate XXXV, fig. 5). If the distal two- thirds 

 of such a hydrotheca were cut off, we would have such a form as Xri-tiilm'rllii hartlaiM (Plate 

 XXVII, fig.' 5). 



(ff) Triangular hydrothecffl. In a few cases hydrotheca' are triangular in outline as viewed 

 from the front, being hounded by three approximately straight sides. This occurs in the />,.*<- 

 moscypkus group of Xr////V//-/</ as .V. /v/-x///y.v/ (Plate I, tig. It), in which the two inner sides of 

 a pair of hydrotheca' are contiguous and pressed together so as to form a straight line, and the 

 aperture at the supero-lateral angle is very small. The triangular outline is also approached in 

 A1>'ni'ni<ii-'ia trask't (Plate XXXIII, fig. lu). 



(h) Perhaps the rarest form of hydrotheca' is one that approaches a parallelopipedon in 

 shape, having a quadrate cross section and four parallel sides. This form is well shown in 

 SaiulitreUa <jn<i<1/'iitn (fig. 23), and in X. <'i/linilr!tln-<-i( (Plate XIX, tig. -4). A combination of 

 this form and the barrel-shaped hydrotheca is found in X /////,//,//,/ //////>.<//. where the upper 

 part of the hydrotheca is modified in shape so as to be square in section, while the remainder is 

 barrel-shaped. The extent of immersion varies greatly in most of these types, but is probably 

 most complete in some species of Tlntiiu-in, as T. ///////<r.w (Plate IX, fig. 4), in which the entire 

 hydrothecal margin is sunk to the general level of the hydrocaulus so as to be flush with it all 

 around. 



There is in general a pretty close adherence to type in the hydrotheca} of a given species, the 

 most notable exception being the case of /'tx////i,i/ <[tiii(lri<l. ntntit (Plate XIII, fig. -I), in which 

 the hydrotheca? are arranged in groups of pairs, no two pairs in a group being alike, the upper 

 pair being smaller than the lower and more or less quadrate in outline. A slight approach to 

 the same condition is found in the case of TJmiiirin tulmUfni'niis. On Plate XI, fig. 2, for 

 instance, the lowest pair of hydrotheca' is much broader from margin to margin than the next 

 pair. Another and more evident example is found in S,-rtnliir!<i mmjiri (Plate V. figs. 1,2), 

 where the hydrotheca 1 on the basal part of the stem are much longer than those on the distal part, 

 and are bent abruptly outward, forming nearly a right angle at their middle portion. 



