300 PROr. P. M. DUNCAN S EEVISION OF THE 



oval, large or small, and eyen deformed and oblique ; it may be supra- 

 marginal, marginal, or infra-marginal ; it has plates surrounding the anus, 

 sometimes numerous, rarely few and triangular. 



Amlmlacruvi, one of the five ambulacra. (See Test, Dorso-central System, 

 Plates.) — Consists of an interporiferous area, which is placed between the 

 two poriferous zones. The ambulacra may be straight, curved, wavy, subsig- 

 moid, flaring, broad, narrow, flush or sunken, according to the usual 

 meaning of the words ; their path is dorsal, ambital, actinal, peristomial, 

 and buccal— these being the regions of the test passed over. They ai-e 

 similar, or, when the anterior one differs from the others in its pores, 

 dissimilar ; they are either Close at the Apex, or I}isconnected when the 

 posterior Eadial plates are disjunct. They are named according to their 

 position, and are : — 1, Anterior or odd ; 2, Antero-lateral, right and left ; 

 3, Fostero-lateral, also right and left. Each ambulacrum being associated 

 with a Eadial plate, assumes the number of that plate in Lov^n's termi- 

 nology. Ambulacra may have their dorsal portions petaloid and the rest 

 simple, except often near the peristome. In the Exocyclica the anterior 

 ambidacrum and the antero-lateral pair are sufiiciently connected to form 

 a Trivium, in conti-adistinction to the postero-lateral pair, which then 

 form a Bivvmn. The termination of the ambulacrum at the Peristome is 

 the peristomial margin of it. The petaloid parts are either flush with the 

 test, or project and are tumid, but most commonly they are slightly or 

 considerably sunken, and in the last instance they frequently are Marsupia, 

 or receptacles for the immature young ; they may be in deep and narrow 

 or broad grooves. 



The areas have primary and secondary tubercles in vertical rows, or 

 the last kind may be irregularly disposed or in scrobicular circles around 

 the primaries (see Plates). Granules occur and Ejyistroma. 



The poriferous zones are on either side of an interporiferous area, 

 except in some Palaeozoic genera, where all the ambulacrum is pori- 

 ferous. The arrangement of the pairs of pores may be in simple 

 series, that is to say one pair is placed over the other from peristome to 

 apex ; or they may be placed so that there are two vertical series, one 

 nearer the ambulacro-interradial suture than the other, they are then 

 termed "biserial" (bigeminal of authors); or there may be three vertical 

 rows of pairs, the arrangement being " triserial" (trigeminal); or there 

 may be a crowded and apparently disorderly arrangement of the pairs 

 near the peristome, and it is "Polgserial." 



Simple series of pairs of pores are either absolutely straight or in arcs of 

 three or more pairs. The innermost, and therefore the lowest or adoral of 

 the pairs of an arc of three pairs, is usually connected with the adoral plate 

 of a compound plate, and when there are biserial pairs the distribution of 

 the three pairs in an arc is closer and wider apart laterally (see Echino- 

 thrix, fig. 7, pi. V. Linn. Soc. Journ. xix.), or as in Micropyga (see same 

 plate, fig. 11). The adoral pair of the triplet is then not always the most 

 internal of the three. (It is clear that there are three pairs of pores to each 

 compound plate of the biserial Micropyga, and therefore the word bige- 

 minal, which used to be the term, is incorrect.) Pairs are said to be in 



