450 PKOF. p. M. DUNCAN ON THE STRTJCTUEE OP 



median line of the ambulacrum by the growth of the inner set, 

 Plate 1 of the compound is in relation to an inner pair of pores and 

 it is the aboral primary. Plate 2 is one of the outer set and it is a 

 demi-plate which only reaches a very slight distance from the pair 

 of pores. Plate 3 is in relation to an inner set of pores, and it is 

 larger than the last, but still it does not reach the median line ; it 

 is a large demi-plate. Plate 4 is a small demi-plate and it belongs to 

 the outer series of poriferous plates. Plate 5 is one of the inner series 

 and is a large primary, the middle one of the three primaries. 

 Plate 6 is one of the outer set and is a large demi-plate or possibly 

 a primary. It is of the usual shape of the adoral primary in com- 

 pound plates situated lower down.. 



In a compound plate with six plates, in Dr. Wright's collection, 

 of which I took a diagram (fig. 25), there is little doubt that the diplo- 



Fig. 25 (see p. 452). 



podous condition must be considered as really affecting the relative 

 dimensions of the plates. In the specimen (C. Kbnigi) the existence 

 of an adoral and aboral primary is evident, and there is a large 

 middle primary ; but it is in relation to the fifth pair of pores. The 

 second, third, and fourth plates are demi-plates, and are arranged 

 after the fashion of Strongylocentrotus, as described by Loven. This 

 is the same arrangement as is seen in the diplopodous compound 

 plate. 



Under the circumstances the Cyphosomoid type of ambulacrum 

 differs from the Biadema-tj^e in its simplest expression, and also 

 from the diplopodous type exemplified in Diplopodia versipora. 



The Cyphosomoid type may be said to unite the Diadematoid and 

 the Echinoid types. 



IV. Conclusions PvElating to the types of Ambulacea. 



It may be now assumed from the results of former observations, 

 and from the consideration of the structures noticed in this essay, 

 that there are certain well-defined types of ambulacra in the regular 

 Echinoidea, 



1. The Cidaroid type. All the plates of the ambulacra are 

 primaries, and they do not combine to form compound plates. 



2. The Diadematoid type. The newest plates are primaries, and 

 at greater or less distance from the ambitus three primaries unite to 

 form a compound plate, the middle plate of the three being the largest. 

 (Journ. Linn. Soc. Zool. vol. xix. p. 95, 1885.) 



3. The Arbacioid type. The newest plates are primaries, and at 

 varying distances from the ambitus three primaries unite to form a 



