GENUS ORBITOLITES. 123 



been at first spiral — were constantly presented in well-marked contrast with each other, 

 there would be good ground for considering them, as Prof. Williamson has done (cviii), 

 to be characteristic of distinct specific types. But this idea cannot be sustained w'hen a 

 large number of individuals are examined and compared. For it then becomes apparent 

 that the number of cases in which the primitive disk is surrounded on all sides by the same 

 number of zones, indicating that the concentric mode of growth has prevailed from the 

 first, are very few ; but that in by far the larger proportion of specimens there is a slight 

 excentricity of the primitive disk, witli a larger number of zones on one side than on the 

 other, as in Plate IX, fig. 3 ; indicating that the first-formed zones have been incomplete 

 circles, owing to a restriction of the gemmation of the circumambient segment to one part of 

 its periphery. This is shown extremely well by decalcified specimens of the animal, scarcely 

 any two of which, in fact, precisely resemble one another as to the mode in which the first 

 zone originates in the circumambient. Thus in the specimen represented in Plate IV, fig. 14, 

 of which the primitive mass is represented on a larger scale in fig. 20, the circumambient 

 segment gives off only three peduncles, at the end most remote from its connexion with the 

 primordial segment ; and the first zone of segments is far from being entire, the cyclical type 

 not being completely attained until two or three successive additions have been made. In 

 fig. 1 9 ei^ht peduncles are seen to be given off from the circumambient segment, and from the 

 half-zone which they form an entire circle is next produced ; thus affording a remarkable con- 

 firmation to the idea I have already suggested (^ 1 75), as to the capacity of a portion of a zone 

 to give origin to a complete annulus, by the lateral extension of its bands of sarcode. In 

 fig. 18 the circumambient segment gives off eleven peduncles on one side, and there are 

 indications of iliree or four on the other. In fig. 17 the peduncles come forth from a still 

 larger proportion of the periphery of the primitive mass ; the zone which first surrounds it, 

 however, is still incomplete in some parts, though the succeeding zone forms an entire circle. 

 In fig. 15 we see peduncles coming off from various parts of the circumambient .segment, in 

 which (as in the specimens represented in figs. 16, 17, 18) there is a partial separation of 

 a secondary segment b' . Finally, in the specimen represented in fig. 1 6, which is almost the 

 exact counterpart of the disk represented in Plate IX, fig. 4, and diagrammatized in Fig. XXIV, 

 the peduncles come off from the entire circumference of the circumambient segment, and the 

 annular zones of segments are complete from the first. The greater the limitation of the 

 power of gemmation to one side of the nucleus, and the larger the number of incomplete zones, 

 the more will the early plan of growth approximate to the spiral type, such as is represented 

 in Plate IX, figs. 3, 5. — It is obvious that the existence of such intermediate gradations 

 breaks down that barrier between the extreme forms, which Prof. Williamson had proposed 

 to erect ; and shows that in this, as in many other particulars, differential characters which 

 at first sight appeared to be perfectly satisfactory, lose all their force when carefully traced 

 through a sufficiently extended series of specimens. 



181. Monstrosities. — Besides those departures from the normal type of growth which 

 have been described as variations or irregularities, there are certain others of rarer occurrence, 

 which can only be regarded as " monstrosities by excess ;" consisting in the production of 

 one or more incomplete secondary disks by outgrowth from the first. Thus in one specimen 

 in my possession the secondary disk forms a half-circle B D, of about the same diameter with 



