B 



124 FAMILY MILIOLIDA. 



the primary a c, and is superposed vertically upon the latter, the plane of junction passing 

 i> through its centre. In other specimens the secondary disk is relatively 



smaller, extending only from the centre to the margin of the primary, but 



"^ s still meeting it nearly at right angles. — In another specimen I have met 



with, it would seem impossible to say which is the primary and which the secondary disk ; 



and it might be more correct to describe the entire structure as consisting of a single half- 



'C disk A B and of two half-disks b c and b d, meeting each other at an acute 

 angle c b u, neither of them being in the same plane with the single half- 

 disk, but both of them meeting it at similarly obtuse angles A b c and a b D. 

 In another case there rises from the surface of the disk a triradiate crest, formed by three 

 vertical plates meeting one another at nearly equal angles, but all of them neai'ly perpen- 

 dicular to the plane on which they rest. It is a very remarkable feature in this specimen, 

 however, that the line in which the three vertical planes meet is traceable at its base to the 

 centre of the horizontal disk ; so that they all bear the same relation to the primitive disk as 

 does the single outgrowth in the instances previously cited. Hence we may attribute all 

 such monstrosities (of which I possess a remarkable collection) to an excess of productive 

 power in the sarcode of the primordial segment, which has put forth its first extension, not 

 merely in the horizontal, but also in the perpendicular direction ; the whole subsequent 

 development of these outgrowths taking place after the normal plan, from the foundation 

 thus laid. I have occasionally met with instances, however, in which a vertical plate rises 

 from the peripheral portion of the disk, at a distance from the primordial chamber. — It is 

 interesting to remark that the presence of such outgrowths as those now described is far 

 more frequent in certain localities than it is in others. Among some hundreds of specimens 

 which I have examined from the coast of Australia, I have only met with five or six ; among 

 those yielded abundantly by the sand of the shore at Suez, such monstrosities are far more 

 frequent, and the excess more pronounced ; but in a small collection which I have inspected 

 from the Mgean Sea, the monstrosities of this kind were so numerous, that I think I am 

 scarcely vsTong in asserting that one specimen out of every three or four presented some 

 excess.* Among the fossil Orbitolites of the Paris basin, the presence of a completely 

 semicircular vertical plate is not at all uncommon. 



182. Essential Characters of Orbitolites, and its Relations to other Types. — If, now, we 

 seek to determine the essential characters of Orbitolites, we find them to lie in the presence 

 of a series of annuli of sareode (and of corresponding galleries in the shelly disk) arranged 

 concentrically round a "primitive disk," which resembles a young Miliola ; each zone in the 

 simpler type containing but a single annulus, so constricted at intervals as to form a series of 

 somewhat columnar segments, which occupy the chamberlets of the shelly disk and are con- 

 nected with each other by stolons of sarcode ; whilst in the more complex type each zone 



* This is by no means a solitary case of tlie prevalence of monstrosities in particular localities. 

 The collection of Mr. Bean, of Scarborough, contains a number of curiously distorted specimens of 

 the common Planorbis marftnatus, which have all been collected in one brook. Their peculiarities are 

 by no means repetitions of each other ; and I am disposed, therefore, to regard tliera rather as 

 resulting from the influence of external conditions, than as accidental varieties hereditarily propagated. 



