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Show -1896.] GENUS OF FOSSIL ECHINODERMS. 1001 II. Description of Lysechinus. LYSECHINUS 1, nov. gen. DIAGNOSIS.-Plesiocidaroida with the ambulacra limited to grooves on the oral half of the test. DESCRIPTION.-Test small and slightly elliptic; margins tumid ; oral and apical surfaces flattened. Apical system very large, and forming most of the test. The basal ring consists of five plates, forming a closed ring. O ne (? more) of these is perforated by a pore. Their form is apparently heptagonal. Ocular plates very large; they are hexagonal; five of the sides are straight, but the sixth is broken by a notch for the end of the ambulacrum. Periproct large; an irregular pentagonal ellipse. Ambulacra.-These occur in five (?) somewhat spoon-shaped depressions around the mouth. There are four or five small single pores on each side of each ambulacrum. lnterambulacra large. Apparently each consists of nine plates ; there is a large single peristomal plate succeeded by two plates, above which are two series each of three plates. The ornamentation consists of granules or small tubercles irregularly arranged. The spines are short, with a stout proximal knob. Peristome very large, occupying nearly the whole of the lower surface of the test. DIMENSIONS.-Height 4 mm. Diameter 7 „ Diameter of periproct .. 1| „ „ „ peristome .. 3^ „ DISTRIBUTION.-St. Cassian Schichten. Trias: St. Cassian, Tyrol. T Y P E SPECIES 2.-Lysechinus Incongruens, n. sp. Brit. Mus., E 3935. III. Affinities of Lysechinus and Classification of the Plesiocidaroida. Tbe interpretation of tbe specimen on which this genus is founded is unquestionably difficult, owing to its small size, to the 1 From \U'IT(S, dissolution or disconnection. In Prof. Bell's ' Catalogue of British Echinoderms,' 1892, pp. 14, 24, the term lissactinic is used as a synonym of azygopodous. This is obviously a printer's error, Xvviv having been mistaken for Xiaaos, smooth. The slip is here corrected at Prof. Bell's request; the word should be " lvsactinic." ... 2 There being only the one species it is impossible to say which of the characters are specific and which generic. N o specific diagnosis is therefore possible. |