5i PROCEEDINGS OF THE ACADEMY OF [March, 



Stage, which begins by the appearance of two swelhngs or tubercles 

 on each rib. One of these tubercles is near the suture, and the other 

 at the position which is later occupied by the shoulder angle. They 

 will be referred to as the suture and shoulder tubercles respectively. 

 These two tubercles are often of nearly equal size at the start, though 

 frequently the suture tubercle is the larger. Tubercles other than 

 these two mentioned are seldom visible on an early rib ; but when a 

 sufficiently small shell is obtainable, other smaller tubercles can be 

 seen upon each rib, and the tubercles diminish in size quite regularly 

 from the suture tubercle downward. Each tubercle is connected with 

 the corresponding tubercle of the next rib by a faint spiral, the first 

 appearance of the definite cancellation which later becomes more 

 pronounced. In most specialized forms the shoulder angle develops 

 shortly after the appearance of the tubercles. 



The cancellated condition is found more or less well developed in 

 all the different races. In primitive species ^ it may persist as a con- 

 stant feature to the end of the individual's life; but in most forms it 

 covers only a few whorls and is more variable than the preceding 

 Curved Rib Stage, in that certain of its features change as the shell 

 grows. These changes are gradual, but they render the end of the 

 Cancellated Stage much less definite than its beginning. They may 

 be briefly summarized as follows : The tubercles, with the exception of 

 the shoulder tubercle, usually tend to degenerate, and soon disappear, 

 though their corresponding faint spirals may sometimes persist. 



The shoulder tubercle, on the contrary, becomes stronger and sharper. 

 First the ribbing above the shoulder disappears, and later the ribbing 

 below the shoulder shortens, and with its practical disappearance 

 what might be termed the Spiny Stage is inaugurated. 



In the Spiny Stage the shoulder tubercle is now sharp and spine-like. 

 Other tubercles have disappeared and the spirals are, as a rule, re- 

 stricted to the region of the anterior siphon. Ribbing has practically 

 gone, but very short rudiments may sometimes still be present below 

 the shoulder spines. The commencement of this stage.is never sharp, 

 for it is the result of gradual change from the preceding one.® It is 



5 In A771. Nat. for 1902, Vol. 36, p. 926, Grabau says: "It is perhaps not too 

 much to say that in the majority of the larger phyletic series, except those highly 

 specialized, the radicle is a smooth, round-whorled form, succeeded by types in 

 which the adults are ribbed, and later cancellated, after which progressive modi- 

 fication may be carried further." 



'No morphological distinction is made between spine and tubercle. "The 

 writer considers that the difference between the small rounded tubercle of the 

 early whorls and the large, sharp spine of the later whorl is one merely of degree. 



