420 PEOF. p. M. DUNCAN ON THE STRTJCTTJEE OP 



the ambulacra can be readily observed ; but the separations of, or 

 the sutures which unite, the edges of the plates are usually not to 

 be seen. Many weathered specimens of Oolitic and Cretaceous 

 species show all the details of the ambulacra ; and when they are im- 

 perfect or slightly confused, the knowledge of the position of the 

 plates in the modern allies of the fossil forms renders the action 

 of reagents almost invariably successful in distinguishing them. But 

 it is not possible to make out the limits of the ambulacral plates in 

 some specimens, and usually these forms are the most perfect in their 

 state of preservation. Weathering and the action of Loven's reagent, 

 alcohol mixed with glycerine, assist the investigation of the fossils, 

 and most liquids which permeate the plates and evaporate rapidly 

 will assist in the investigation of recent Echinoidea. 



II. Genebal Eemaeks on the STRTJCTirEE OF Ambtjlacra. 



An ambulacrum extends from the actinal edge of a radial 

 (ocular) plate to the edge of the peristome; it is composed of a 

 number of plates which are placed in two rows or zones on either 

 side of a vertical or median line, and each row consists of plates 

 located in succession from the radial plate to the peristome. The 

 plates of one row are united by suture with those of the other, and 

 the junction occurs along the median line, the extremities of the 

 plates being more or less angular. The plates of one zone are not 

 on an exact level with those of the other, for the angle of one plate 

 fits into the reentering angle which exists between two of the 

 opposed plates. The edge of each plate in contact with the inter- 

 radium is also more or less geometrical, but it may be rounded off 

 in shape, instead of angular. The plates in each row are united 

 actinally and abactinally with other plates. These unions are by 

 suture. 



Each plate has a poriferous zone, and the rest forms part of an 

 iuterporiferous area ; and there is a pair of pores to a plate. In well- 

 preserved specimens the pair of pores is encircled by a raised dish- 

 like structure called a peripodium, and the pores are never placed 

 quite transversely or horizontally, but more or less obliquely, so 

 that the pore nearest the median line of the ambulacrum is on a 

 lower level than the other ; that is to say, the inner pore is adoral 

 to the other and is called the adoral pore. This pore is in contact, 

 in the young Echinoid, with the line of division between two con- 

 secutive plates, and notches the edge of the peripodium ; but it may 

 become distant from the suture during growth. In the Cidaridse 

 aU the plates are primaries, and each has its pair of pores. A 

 primary plate is one which reaches from the poriferous zone to the 

 median line or vertical suture, and which comes in contact there 

 with others of the opposite zone. In the Cidaridse the plates in- 

 crease in number by developing just at the actinal edge of the 

 radial plate, and one plate is formed after another. 



At the peristomial end of the ambulacrum there is a corresponding 



