208 FAMILY GLOBIGERINIDA. 



the Operculine type as to be readily mistaken for a member of that series, its " truncatuline" 

 character being only made evident by a comparison of the gradational forms by which it is 

 connected witli the type. The Trinicatnl'ma of northern seas is seldom observed to present 

 any more considerable departure from the ordinary Rotaline plan of increase ; but that of 

 the Mediterranean continually takes on a " wild " growth, its later segments being added 

 without any regularity, though usually in such a manner as to tend towards a cyclical ar- 

 rangement ; and specimens of this kind are very commonly found attached to the larger 

 bivalves (such as P-inna flabellum) in company with PlanorbuUna mediterranensis, exceeding the 

 latter in size, though not developing nearly as many chambers. In tropical seas the arrested 

 " truncatuline " forms are rare ; the exuberance of increase manifesting itself very early in 

 the transition to the " planorbuline '' type, on which a number of new chambers may be 

 formed at once, instead of in the succession required by the spiral plan. 



360. The type which has been distinguished by D'Orbigny as Anomnlina may be de- 

 scribed as a modified Truncatulina of which the two surfaces are nearly equal, so that the 

 spire comes to have almost a nautiloid character, and presents a broad sunken umbilicus on 

 either side. The apertural fissure does not show itself on either surface, being limited to the 

 septal plane ; but is nearer to the flatter side than to the other. This form may either become 

 "planorbuline," or may graduate into the "truncatuline;" so that its differential characters 

 are obviously of no more than varietal importance. One of the forms into which it passes 

 has been distinguished as F. {Anomatina) coronata ; a variety less common than P. [Truncatu- 

 lina) lobatula, but abounding in certain localities (as on the coast of Norway and in parts of 

 the Mediterranean) at depths of 100 fathoms, where it lives independently instead of para- 

 sitically, developing both its surfaces more or less freely. Both, in older specimens, are over- 

 laid by an exogenous deposit of clear, non-perforated, shell-substance, which sometimes hides 

 the septal lines ; but in this there exist lacunce for the exit of the pseudopodia, which seem 

 especially to extend themselves from the periphery of the chambers ; and these lacunae seem 

 to prefigure the canal-system which we shall find to constitute a remarkable feature in the 

 exogenous skeleton of Calcarina. 



3G1. It is in tropical regions that PlanorhuUna acquires its largest size and its most 

 marked development, examples of the parasitic P. vulgaris not uncommonly attaining a 

 diameter of l-4th of an inch, and presenting such a resemblance to Polyzoa as to be easily 

 mistaken for members of that group. This augmentation of size partly depends upon the 

 greater multiplication of segments ; but is partly due to the dimensions which the individual 

 segments may attain. The early growth of this type (Plate XIII, fig. 13) closely corresponds 

 with that of P. mediierranensk ; but as its development proceeds we find a remarkable 

 change in the condition of the apertures of communication between the chambers. These 

 apertures, which in the Plunorbulino! of temperate seas are never more than faintly lipped, 

 here become protrusive, each being furnished with a neck and an everted rim. As a general 

 rule we find the pairs of these necks put forth from two adjacent marginal chambers con- 

 verging towards each other, both being received into the new chamber which is formed in 

 alternation with them (fig. 14, «, «) ; and the new segment in its turn puts forth two similar necks 

 from near its peripheral margin, to be received into two separate chambers of the succeeding 



