A treatise on zoology . s and impressions of kind permission of Dr. C. D. Walcott. thecal apertures, one side tending to be convex, the other of the two sides enclosed by a common frame of marginals. Platesof concave side tend to be fewer and more regular than those of convex,but never achieve bilateral symmetry as do the latter. Tapering stem ofpolymeric columnals at one end of theca ; at the opposite end are theapertures, with function still uncertain. In some genera, spines (armsof most writers) are known, one at each upper angle of the theca. Orna-ment o
A treatise on zoology . s and impressions of kind permission of Dr. C. D. Walcott. thecal apertures, one side tending to be convex, the other of the two sides enclosed by a common frame of marginals. Platesof concave side tend to be fewer and more regular than those of convex,but never achieve bilateral symmetry as do the latter. Tapering stem ofpolymeric columnals at one end of theca ; at the opposite end are theapertures, with function still uncertain. In some genera, spines (armsof most writers) are known, one at each upper angle of the theca. Orna-ment of granules, which on the theca tend to run in transverse, wavy,sub-parallel lines, simulating the scale-markings of some Crustacea. Nopores. J. Walther (1886) and Haeckel (1896) have considered the bilateralsymmetry primitive, and homologous with that of the Dipleurula ;but M. Neumayr (1889) maintained that the symmetry of the two wasdifferent. The evidence suggests that the evolution was towards greater 50 THE CYSTIDEA. bilateral symmetry, and therefore started from, the usual sack-like form. Genera—Trochocystis, Barrande (1859-87 ; syn. Trigonocystis, Haeckel), Cambrian, Bohemia, France, Spain (Fig. XL), isthe most primitive ; its theca is bounded bytwelve stout marginals (mm), conspicuous on oneside more than on the other, and forming acircular, elliptical, or subtriangular frame ; theenclosed space on either side is occupied by amosaic of 80-160 somatic plates, hexagonalexcept where truncated by the marginals. Atthe oral end of the frame three openings, nearlyin the broad median plane, pass from the thecalcavity through or between the marginals to theexterior; the middle opening (? hydropore andgonopore) is widest, and is protected by ahood-like projecting plate (M), (^ madreporite) ;of the other two openings, one (0) in the broadplane is the wider (? mouth), the other (As)slightly out of that plane is smaller (? anus) ;they appear to be connected by a canal (? forrecepti
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