A treatise on zoology . mouth and anus. Family 2. Comarocystidae. Rhombifera inwhich thecal plates are numerous and inde-finitely arranged. Radial structure of stereomstrongly marked, but no definite j)ore-rhombs orpectinirhombs. Food-grooves on free exothecalbrachioliferous processes. Columnals in a singleseries. Genera—Gomarocystis, Billings (1854),Ordovician, Canada. Theca ovate, may be over7 cm. high, composed of about 150 mostlyhexagonal plates, wdth strongly marked radialstriation of the stereom, especially towards the margins, which are raised above the umbones dinarins, Auctt.). (After
A treatise on zoology . mouth and anus. Family 2. Comarocystidae. Rhombifera inwhich thecal plates are numerous and inde-finitely arranged. Radial structure of stereomstrongly marked, but no definite j)ore-rhombs orpectinirhombs. Food-grooves on free exothecalbrachioliferous processes. Columnals in a singleseries. Genera—Gomarocystis, Billings (1854),Ordovician, Canada. Theca ovate, may be over7 cm. high, composed of about 150 mostlyhexagonal plates, wdth strongly marked radialstriation of the stereom, especially towards the margins, which are raised above the umbones dinarins, Auctt.). (After An- . eliii.) X -. of the plates. Mouth-slit transverse, with a ° pair of uniserial brachioliferous arms at either end. Theca flattened in mouth-plane. Anus below arms on right side. Hydropore above level of anus, near the posterior margin of the mouth. Stem longer than theca; columnals low, circular, with moderately wide lumen. Acliradocystis, Volborth (1870), Ordovician, Russia, appears to have an anal pyramid of. Fig. Angdini (C. testu- 56 THE CYSTIDEA seven plates, and a very highly developed stem. Thecal plates havestrong radiating ridges, marginal concentric ridges, and suture marginstoothed on the inside in correspondence with the ridges. Since this struc-ture is not unlike that in Comarocystis, the genus is provisionally placed 3. Macrocystellidae. Rhombifera in which the thecaconsists of three or four circlets of plates, subjected to a more or lessregular pentamerism. The stereom is strongly radiately ridged or folded,but no so-called pores or pectinirhombs are developed. Brachiolesare borne by the upper circlet of plates, and within these there may havebeen tegminal plates over the mouth. There is perhaps no intimateconnection between the two genera referred to this family. But theyare both Cambrian, and show an early development of that tendency toreduce the number of plates, which eventually evolved the Glypto-cystidae from a differe
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
