90 A. EARLAXD ON ORBICULINA ADUNCA. 



features Avhich can be considered common to all the specimens of 

 this genus, which is subject to the widest variation in every minor 

 feature. To such an extent is this the case that the single species 

 to which the genus is now restricted has at one time or another 

 been described under about fifty different names. 



The third genus is the subject of my note, Orbiculina, and as 

 with Peneroplis, the genus contains but a single species. Briefly 

 Orbiculina may be described as a Peneroplis in which the segments 

 are subdivided into chamberlets by the formation of internal 

 septa or partition walls. Typically it is a planospiral porcella- 

 nous shell, starting from an original primordial chamber, the 

 successive segments being spirally arranged, and the first few 

 convolutions being usually more or less embracing or nautiloid 

 in shape. 



This nautiloid form, although generally constant in the early 

 stages of the for am, does not as a rule continue after the test has 

 reached a certain size, and the variation which then sets in is 

 liable to modification in two difierent directions. On the one 

 hand the successive segments, increasing very slowly in size and 

 leDgth, depart from the original spiral arrangement, and form a 

 more or less curved linear series of chambers, attaining in extreme 

 cases a crosier-shaped test which is akin to the Spiroline varieties 

 of Peneroplis. On the other hand, the chamberlets in the successive 

 segments may increase so rapidly in size, especially in length, that 

 each successive segment will considerably overlap its predecessor. 

 The result of this mode of growth is that in time the later seg- 

 ments completely encircle the earlier ones, and the foram assumes 

 at first an ellipsoidal and finally a perfectly discoidal shape, with 

 a ring of oral apertures completely encircling the margin. The 

 method of growth now continues annular, and the specimen 

 resembles an Orbitolites except for the fact that the nautiloid 

 early chambers are nearly always distinguishable as a convexity 

 in the central region of the disc. 



Now the most striking feature noticeable in the examination of 

 the large number of specimens which has come under my observa- 

 tion is the great disproportion in size and robustness of growth 

 which exists between the crosier-shaped and the discoidal varieties. 

 As will be seen from the specimens under the microscope, the 

 crosier-shaped variety is less than —^ of an inch in length, while 

 discoidal specimens ^ inch in diameter are comparatively frequent. 



