- SYSTEMATIC SECTIONS. 63 



(2) Changes connected with tJie Lateral and Apical Surface. 



The method of increase of the apical surface is also very characteristic of these 

 older families, being brought about largely by the development of a considerable 

 number of rows of plates between the supero- and infero-marginalia of the base of 

 the arms. The added plates are known as intermarginalia. Schondorf ((32, p. 95), 

 who studied the Devonian Xenasteridae, was the first to draw attention to this 

 intermarginal area, and Schuchert has traced its gradual development in the earlier 

 Mesopalgeasterinte and the Promopala^asterinae. 



Intermarginalia are only rarely present in Recent forms. It is interesting to 

 note that the Cretaceous family of Stauranderasteridge have the area well developed. 



(B) Changes in the Appearance of the Plates. 



(1) Clainges in the Margin alia. 



As is well known, the Recent Asteroidea are commonly divided into two great 

 divisions, the Phanerozonia (Sladen) and the Cryptozonia (Sladen). The Phanero- 

 zonia are the more primitive, and are distinguished by clearly defined marginal 

 ossicles and by a more unbroken calcareous covering. The Cryptozonia are 

 derived from the Phanerozonia since they pass through a Phanerozonate stage 

 when young, but when the forms become adult the marginal ossicles can no longer 

 be clearly separated from the remaining ossicles. This is undoubtedly due partially 

 to the fact that the marginalia are cut away to provide additional lung-surface by 

 exposure of the skin blood- vascular system. 



We can notice among some of the older Asteroidea a similar reduction in the 

 size and importance of the marginalia. They do not grow proportionately as the 

 members of the lineage series increase in size, as may be seen if we compare the 

 supero-marginalia of a " Hudsonaster'" (Text-fig. 36, p. 74) with those of a Pronio- 

 ■palseaster (Text-fig. 51, p. 93). A comparison of these same two text-figures 

 shows that, while the most proximal supero-marginalia are the largest in the more 

 primitive form, in the later form this is not the case. The first pairs of supero- 

 marginalia which have been pushed from a marginal into an adapical position by 

 the development of the intermarginalia, are reduced and are often not easy to 

 distinguish from the remaining plates of the disc. The supero-marginalia also 

 change in shape. Those of the Middle Ordovician " Hudso7iaster," as described by 

 Schuchert (85, pi. v, fig. 2), are polygonal and straight sided, while those of the 

 majority of the Upper Ordovician and later forms have their proximal inner margin 

 cut away so that the ossicle is shaped like a bent finger. The shape of these 

 plates is very well seen in Text-fig. 34, p. 71 {Girvanaster sculptus). Here 

 undoubtedly the spaces between the successive supero-marginalia have some con- 



