10 ANj\ALii \f:\\ lOA'A ACADEMY OF SCIEXCI'JS 



oxpaii.-ion Ileal- tlir |)uiiit (if ciij-Nal lire and tticir dorso-vcntral flattening. 

 In till' aims of ('. rd iiiihil us. ilic stage of the cylindrical snioulli arm is 

 followed 1)\ one ill which each |date bears a short jjroniiju'iit ridge ni-ar the 

 lateral margin which i> helieved lo represent tlie eonHnent nodes of its ;in- 

 eestor, now reduced to one elongate node, or short ridge, placecl close lo tlu: 

 lateral iH\<rp of the jdate. A i'ew plates later/a^rnall node appears near the 

 median line. This stage is represented on Plate 111. fig. .'!. In successive 

 plates (tf the arm, these two nodes api)ear nearer and nearer to the lateral 

 margin until a plain s]»aee is left into which a new row of nodes is intro- 

 duced near the median line. The median nodes increase in streng-th, while 

 the (Uiter ones diminish in size as they recede toward the lateral margin 

 until they disappear. Meanwhile, the form of the arm has changed, be- 

 coming flattened dorso-ventrally so that the former median node occupies 

 the lateral margin and is elongated into a. spine. The line of nodes of 

 latest origin remains near the median line. These changes are illustrated 

 on Plate 111. figs. ;5, 4. \a-\c. It thus appears that, in this group at least, 

 new features arise near tlie median line of the arm and on successive 

 plates seem to moNc laterally until they disappear and are replaced by 

 features of later origin. This fact has led to the conclusion, stated above, 

 tliat the elongate node on early plates of Cactocritnis rcfii ulaliis repre- 

 sents confluent nodes in a late stage of their evolution and soon to disa])- 

 pear, rathei" than that it has any relation to the transverse ridge present in 

 early stages of its predecessor, C. plali/hrarhiaius. The smooth space thus 

 left on the nie<lian half of the plate, in the preceding species, becomes a 

 field for the introduction of new features which appear successively as 

 lines of nodes. 



In the aiMii (d' <'. dciiluiihil us. the earliest stage to appear is that with 

 an elongate node and a shorter one. both near the lateral margin of the 

 plate. The ancestial features are, in this species, somewhat obscuicd by 

 the presence, on early plates, (d' nodes which are the continuation over 

 the base of the arms of the irregularly ])laced nodes present on the calyx 

 of this species. They constitute a feature of later origin qnite distinct 

 from the two nodes near the lateral margin of the* arm plates. These 

 irregularly placed nodes are present only on early plates of the arm, and 

 by the time the eighth plate is reached, they have disappeared, as shown 

 on Plate IV, fig. 1 . The two nodes remaining after the disappearance of 

 the irregularly phn vd nodes are, in the specimen figured, sharply pointed, 

 but this is not a constant feature for the species. Beyond this point, the 

 evolution of the arm for the greater portion of its length is the same as 

 that described for C. rclkulaius, except that it is more accelerated, new 

 features appearing at an earlier poricMl than in the latter .species. The 



