r . . .. , BRYOZOA. 103 



Introduction.] 



f 



7. The frondescent or palmate zoarium is a modification of the ramose, differing 

 from it in the flattening and expansion of the branches. Examples are Heterotrypa 

 frondosa d'Orbigny and Homotrypa fiahellata Ulrich. 



8. The jointed zoarium, in which it is divisible into a greater or less number of 

 subequal segments, that articulate with each other either terminally or by means of 

 lateral sockets ; is illustrated in Helopora, Arthroclema and Arthropora. 



9. The fenestrated or inosculating zoarium, as in Fenestella and Pkylloporina. 

 The pinnate zoarium is a modification in which the parts of the fronds are feather- 

 like in their arrangement. Pinnatopora and Acanthocladia are examples. 



If the specimens under investigation fall under any except the last two modes 

 of growth, they will probably exhibit either groups of cell-apertures larger than the 

 average, commonly raised above the surface and therefore known as " monticules "; 

 or clusters of small cells or smooth spots called "maculae." The monticules may be 

 rounded, low or conical, and sometimes ridge-like. Examples are shown on plate 

 XXII, in figs. 1, 13 and 24. Frequently there is a combination of large and small cells 

 as .in figs. 18 and 19 of the same plate. True maculae are best developed in the 

 Fistutiporidce, in which they consist of aggregations of lenticular vesicles, but on 

 plate XVI, fig. 6, is a good example of the kind in which the cells are tubular. The 

 non-poriferous margins, so common among the bifoliate Bryozoa, are most probably 

 a modification of the maculae ; see plate VIII, fig. 19. 



The presence or absence of interstitial cells between the ordinary zocBcia, and 

 the determination of their character when present, is the third step in our investi- 

 gation. These cells may be of the nature of " mesopores " — small, closely-tabulated 

 tubes, as in Prasopora and Callopora (plates XVI, XXI, and XXII), or the interspaces 

 may be occupied by "vesicular tissue" as shown in fig. 4 on plate IX, a vertical 

 section of Pachydictya frondosa. The zoarium of Monotrypa is characterized by the 

 complete absence of both mesopores and vesicles (see plate XXVII, figs. 24-29). 



Important diagnostic characters are to be observed in the character of the 

 mouths of the zooecia. They may form short tubular projections (plate II, fig. 6), be 

 enclosed by a smooth rim or peristome (plate I, figs. 17 and 28), or the rim may be 

 minutely papillose (plate XIV, fig. 22) ; or the mouth may be depressed and situated 

 in a sloping area (plate X, fig. 24). Other conditions, described by the terms " direct," 

 "oblique," and "confluent," are obvious without the citation of examples. 



Of other external features, the arrangement of the zooBcial apertures, and the 

 character and marking of the interspaces should be noticed. 



In the further progress of the investigation, which is now carried on chiefly by 

 means of thin sections, it is necessary to determine the presence or absence, and the 



