106 K. ETHEEIDGE, JUN., ON ASTROCRINITES. 



line or crest with several rows of peculiar tubercles (figs. 21 & 

 22). A few examples of these plates have been obtained by Mr. 

 Bennie, in the manner previously explained, separate or united 

 to their "spearhead" plates (b, fig. 8). So far these are the 

 plates of the three symmetrical lobes visible on the ventral sur- 

 face of the fossils ; whilst the points of junction of the crested and 

 tuberculated plates just mentioned with the succeeding pieces 

 forming the dorsal surface are the most acute or prominently ex- 

 tended portions in the general periphery (c, fig. 1). These three 

 crested plates, to the convexity of which the fossils owe much of 

 their lob ate character, are united with the truncated apices of four 

 other plates, which enter into and chiefly form the dorsal surface, 

 and with which they alternate. Two of these are arrowhead- 

 shaped or forked plates, and alike (e, fig. 3 ; b, fig. 6 ; figs. 7 & 7*); 

 whilst the other two appear to be different ; but of their exact form 

 I am not at present quite satisfied. A number of the forked plates 

 have been picked out by Mr. Bennie (figs. 7 & 7*); they are 

 slightly concave internally, with, on each side of the fork, two 

 notched or angular shoulders (b, fig. 7*) and a central concave de- 

 pression (a, fig. 7*), into which the apex of the pseudambulacrum fits. 

 Each half or fork of the forked plates is convex (c, fig. 7) ; and the 

 lateral union of every two contiguous halves gives rise to the lobes, 

 which are continuations of those formed by the crested and tuber- 

 culated plates on the ventral surface (6, fig. 8). The closely packed 

 tubercles on the dorsal surface usually hide the sutures between these 

 forked plates ; but they are visible in some specimens (b, fig. 3, 

 a, fig. 6). As previously stated, I can with certainty only assert 

 that two of these plates forming the dorsal surface have the form 

 represented by fig. 7 — the other two (a, fig. 10), bounding the pseud- 

 ambulacra bordering the un symmetrical lobe, being apparently 

 different in form. In A. tetragonus the Messrs. Austin describe a 

 dorsocentral plate ; but I am again uncertain whether the present 

 little fossils possess one, although in some specimens there are 

 certain small grooves amongst the tubercles which appear to mark 

 the outline of such a plate. If a dorsocentral plate is present, then 

 it is to be seen in fig. 12, where lateral pressure has thrust it up 

 out of its place (a, fig. 12). 



Passing now to the fourth or unsymmetrical lobe, we meet with 

 a much more complex structure, exceedingly well shown in fig. 5. 

 The surface is much more flattened than that of either of the other 

 three lobes (b, fig. 5), and occupied by an elongated pyriform aper- 

 ture (fig. 5 ; a, fig. 20). On one side of the latter is an elongated 

 plate (c, fig. 5 ; 6, fig. 20), the ambulacral extremity of which has 

 much the appearance of being half one of the " spearhead " plates 

 (a, fig. 5). If this view is correct, the corresponding half on the 

 other side (c, fig. 20) would occupy, during life, what is now a 

 vacant space (d, fig. 5) merely filled with matrix. These two halves 

 partly bound the excentric aperture, the remainder of the latter 

 being excavated out of another plate (e, fig. 5; figs. 13 & 14; cZ, fig. 20), 

 triangular or somewhat deltoid in form, and broader than long, 



