| OCR Text |
Show 1895.] MR. F. A. BATHER ON UINTACRINUS. 979 The crown consists of a relatively large, globose calvx and 10 long arm-branches (PL LV.). The calyx was flexible, as shown by the thinness of the component plates, the absence of fractures, and the regularity of the preserved specimens. The dorsal cup alone is known to us. The tegmen was pliable, and probably but slightly, if at all, plated; at any rate, none of the ventral plates are exposed, " nor," as says M r . Hill (9), " has it been possible to expose them by dissecting away the plates." Neither does a transverse section of a calyx, at about the level of the 6th fixed distichal, show more than a confused calcareous mass. The dorsal cup (see figs. 4 and 5) consists of three categories of elements : (a) the apical system ; (6) the brachial elements, which are radial and primary ; (c) the secondary, supplemental plates, viz. (i.) interbrachials, (ii.) interdistichals, (iii.) interpin-nulars. The apical system (PL LIV. fig. 1) consists of (i.) the centrale or central apical plate ; (ii.) 5 interradially situate basals surrounding it; (iii.) 5 radials succeeding the basals. The centrale is pentagonal, but in specimens examined not quite regular. In specimen e its greatest diameter is 1*5 m m . It is perfectly smooth, showing no signs either of a stem-attachment or of partition into more than one original element. Its homologies are therefore doubtful, as its structure and position permit it to represent either a relic of a stem, or a fused infrabasal circlet, or even, as some would have it, an additional element altogether to which the name " dorsocentral" might be strictly applicable. I have recentlyl given reasons for rejecting the term and the idea " dorsocentral." Which of the other alternatives be correct is to be decided, if at all, by reference to the affinities and origin of the genus, as to which w e are, at this stage of the inquiry, quite in the dark. The basals (PL LIV. fig. l)are 5, equal, regular, and pentagonal. They surround the apical plate and meet each other by adjacent sides. The sides enclosing the upper angle of each basal are slightly curved convexly, thus giving the basals a petaloid aspect. The measurements of the basals in specimen e are : height 3*4 mm.; width below, 1*0 m m . • width above, 3*5 m m . The radials (fig. 5 and PL LIV. fig. 1) are 5, equal, heptagonal or hexagonal according as the upper sides of the two basals on which each radial rests make a reentrant angle or a straight line. They meet each other by adjacent sides, support the first primi-brachs above, and abut on the proximal interbrachials on either side. Their measurements in specimen e are: in one radius, height 5*0 m m . ; width below, 4*9 m m . ; width in middle, 8*7 m m . ; width above, 5*75 m m . : in another radius, height 6*0 m m . ; width below, 4*5 m m . ; width in middle, 8*0 m m . ; width above, 4*5 m m . The fixed brachial elements that enter into the composition of 1 " The Text-book Writer among the Echinoderms," Natural Science, vol. vi. pp. 415-423 (1895). |
