MUSEUM OF COMPARATIVE ZOOLOGY. 135 



ambulacral series, there are two pairs which may be considered in the 

 account of the plates of the actinal region of the disk of the young Am- 

 phiura. The former of these, or the adoral pair, are known as the spoon- 

 formed plates {spl) ; while the second, partially concealed from view in 

 some of my figures, form the ossicles which complete the calcareous ring 

 of the mouth. The spoon-shaped plates may be known (following Ludwig) 

 as the first pair of ambulacral ; the second as the second pair of ambula- 

 cral. The former are superficial in their position ; the latter more pro- 

 found. Both resemble each other in their paired origin. 



Spoon-shaped Plates. — If we study a young Amphiura in which the 

 pentagonal form has been donned (Fig. 15), there will be noticed in the 

 radius near the mouth opening and between adjacent pairs of first adam- 

 bulacral plates two elongated reticulated plates placed side by side. These 

 plates are commonly pointed on the adoral ends, more spatulate at the 

 other extremities. They originate early in the growth of the embryo, 

 and in young stages are equal in size to the first pair of adambulacral. 

 The increase by growth of the first pair of adambulacral plates leaves, 

 however, the spoon-shaped plates smaller than the adambulacral. Their 

 position leads to the theory that they belong to the ambulacral series, and 

 their mode of origin does not disprove what their position teaches ; but it 

 may be doubted from their want of connection with the first pair of 

 adambulacral plates in early conditions {vide infra) whether they are 

 "ambulacral plates." 



Second Pair of Amhidacral Plates. — The aboral radial ends of the first 

 pair of adambulacral plates of adjacent pairs of these structures are knit 

 together by two plates which are classed as the second pair of ambulacral 

 plates. These plates form the radial part of the circumoral calcareous 

 ring, and occupy the same position as regards the first pair of adam- 

 bulacral plates as the arm-joints do to the side plates of the arms. 



That the second pair of ambulacral plates are ambulacral in their 

 homology is not doubted, but the fact of their union with the first pair 

 of adambulacral has led me to doubt whether they were not the first 

 pair of ambulacral instead of the second. There seems much to support 

 this theory, for it brings all the ambulacral plates whether of the disk or 

 arms into harmony ; viz., an ambulacral plate is joined to a correspond- 

 ing adambulacral. The spoon-shaped plates not only show an exception 

 in their superficial position, but also in early condition are not joined to 

 the first pair of adambulacrals. Although for convenience they are in 

 this paper designated as the second pair of ambulacral plates, it is not 

 wholly clear to me that they are not in reality the first pair, and that 



