18 AMERICAN HYDROIDS. 



Again, it often happens that an oval aperture seems to he pinched, as it were, on opposite 

 sides at the ends of its long diameter. This produces what is known as the angulated margin, 

 such as is found in several species of X A/////;</AS-/.V, as .V. pinnata (Plate XXXIX, tig. H), and is 

 very difficult to distinguish from certain margins with two teeth, when looking directly into the 

 aperture, as in TJuiinrin t,'>icrn (Plato XI, tig. 11). A lateral view of the same hydrotheca, how- 

 ever, discloses the fact that the margin is bidentate (Plate XI, fig. 10). 



Among American species of Sertularidse the number of teeth never exceeds four in normal 

 hvdrotheca-. There are several Australian forms, however, in which the dental armature is 

 much more complicated, there being sometimes as 11111113" as sixteen, as in Sertularia acantJwstoma 

 Bale. 1 In designating the position of the teeth it is customary to speak of those on the side of 

 the margin nearest the hydrocaulus as "adcauline," those on the opposite side as "abcauline," 

 and any situated about midway between these points as ''lateral.'' 



Hydrothecse with a single marginal tooth are rare, the examples being practically confined 

 to the genus Tlnilm'lii. as T. clctjnnx (Plate VII, fig. 4). In this case, however, there is a very 

 large abcauline tooth and the adcauline margin is so closely appressed to the hydrocaulus that it is 

 difficult to tell whether there is an adcauline tooth or not. In T. kurilx (Plate IX, fig. 1) there 

 is a single very conspicuous adcauline tooth. 



The bidentate margin is very common in the Sertularida?, and is in general characteristic of 

 the genus Sertularia. The teeth are usual! y lateral and opposite, and appear often as if a tubular 

 hydrotheca had been beveled on the adcauline and abcauline sides of the distal end. These oppo- 

 site lateral teeth are often quite unequal in size, as in the case of Tlniutriit niyoifni. (Plate XII, 

 fig. 4) and T. ilffinxa (Plate X, fig. 2). But in man}' other cases they are proxiinally of equal 

 size, as Tliulnrlit j>l it until fan (Plate IX, fig. 9). Sometimes the two teeth are both abcauline and 

 very conspicuous, as in Sei'tuliu'clln i'j>!x<-tij>nx^ X // nliii'ia 'operculata (tig. 28), or Abietinaria i/rcenci 

 (figs. 2!*, 84). In this latter case we have perfectly even margins on the hydrothecje on one part 

 of the colony, and two strong marginal teeth on those of another part of the same colony. 



Three marginal teeth are found in many species of Sertularella, and a few in Sci'tnl<n'!<i and 

 Thuiaritt. In Sertiilivll<i they are usually equal in size and equidistant from each other, and 

 vary from almost imperceptible prominences on the margin to pronounced pointed teeth that 

 form a very striking ornamentation, as in Si'i-tiiliircllit phntata (fig. 27), A trii-iixpirfutii (figs. 30, 35), 

 and A'. Jill/arm Is (fig. 31). Rarely there is a small adcauline tooth and two large and conspicu- 

 ous abcauline teeth, as S, rtiil<n;-lla titrgidn (Plate XXII, fig. 3). In the genera Sertularia and 

 Tlniiitrin the three teeth, when present, differ greatly in size, there being two large opposite 

 lateral teeth, and one very small adcauline tooth, as in Sertularia rathbuni (Plate III, fig. 9), and 

 Thuiarla tubuKformis (Plate XI, tig. 5). 



Four marginal teeth are found in the genus SertnlnrcUa alone, and their variations are about 

 the same as those just mentioned in connection with three-toothed forms. The}' are often so low 

 and inconspicuous as to be difficult to make out, as in S. lata. (Plate XVIII, fig. 10), and 8. pinni- 

 (jera (Plate XIX, tig. 3). These very low teeth are apt to be associated with practically complete 

 immersion of hydrothecse, as shown in the figures just cited. In only a few cases are they veiy 

 pronounced and conspicuous, and then they are apt to be unequal in size, the abcauline pair 

 being the larger, as in S. contorta (Plate XVIII, fig. 7). 



It has been suggested by Hartlaub that the hydrotheca is lined by an epithelial membrane. 2 

 In a certain species of Sertrulcvrella this writer found a membrane with a large central opening 

 stretched across the aperture of the hydrotheca, as a velum is stretched across the bell opening of 

 a medusa. From a study of this specimen, and also from the fact that empty hydrotheca? often 

 present certain shrunken structures fastened around the inside of the margin, and from the 

 presence in many species of a ring-like line just below the margin and running around the 

 hydrotheca, this writer suggests that the hydrotheca 1 have a thin epithelial lining which some- 

 times discloses itself in empty hydrotheca 1 in the form of a shallow funnel-like sack attached to 

 the hydrotheca along the ring-like line referred to. 



'Australian Hydroid Zoophytes, 1884, p. 85, pi. iv, figs. 7 and 8. 2 Revision der (sertularella-Arten, 1900, p. 11. 



