No. 490] 



ORTHOGEXETK ' I M /// 1770.V 



noticeable, though recognizable to ilu- toucli as a faiiil protuber- 

 ance. From this beginning the ,s|)iii(> ni'ow >, period In- j >crio<l. until 

 it has become of the normal adult dinicnsious. In vomc forms 

 the spine continues to grow throuu'lifnit litV. i:i\iMi:' ii> tlic enor- 

 mous spines of Fuh/ur candelcdtrinii . 



Where a second row of spines appear^ diis always coiiio in laier 

 than the first, though acceleration may tend to sliortcn uj) the inter- 

 val, or even by dropping out the earliest stages produce a type 

 ill which more than one spine appears at the beginning of the 

 conch stage. Tiidicula is a good example of normal succession 

 in the apijearance of spine rows. In the Miocene T. rvsllcida, 

 only a single row of fulguroid spines appears on tlie anibital iuiu'u- 

 lation (Fig. 3). In T. h{spi7iosa sp. nov.' ( Fius. 1, :! and h the 

 spines of the first row increase in size, progressively tlioiiu'li slowly 

 while a second row of spines makes its ap{)earaiice lower dow n on 

 the body whorl, after the first has been in existence foi- a conMiler- 

 able number of volutions. This second row begins with the in(>re>i 

 elevation, scarcely visible though noticeal)le to the touch ( Figs. 1 and 

 3). From this beginning the size of the spine increases period by 

 period, until the spines of the second row are eciual in size to those 

 of the first (Fig. 4). In accelerated tyi)es, the second row is already 

 ^vell developed in the youno; individuals (Fig. 1).- In the modern 

 Tudicdn .^pi rill Its, a beginning of the second row of spines is indi- 

 cated by a snce(vssion of faint elevations, each later one stronger 



When more than't^^o ^pine. or( nr. the.e follow the same rule, 

 as may be ^een in Munx hnwdaris, M t,nu,^,>nui, etc. 



In all cases the si>iues a[)])ear on the spirals, representing periodic 

 outgrowths of the emarginatioji which originally formed the s])iral. 

 As already noted, the first spine occurs on the spiral oceujiyino' 



