North American index fossils, invertebrates . 589 XV. Cyphosoma Agassiz. Test depressed, highly ornamented. Ambulacra with well-developed pore-bearing zones, undulating, and each plate with threeto seven pairs of pores in an arc. Pairs of pores in two rows atthe apex and crowded at the mouth opening. Interambulacra withtwo or more vertical rows of primary tubercles which are imper-forate and crenulate, like those of the ambulacra. Apical systemencroaching upon the posterior interambulacrum. Jurassic-Recent. 23. C, volanum Cragin. Comanchic. Small, similar in form to Pseudodiadema texanum; pore
North American index fossils, invertebrates . 589 XV. Cyphosoma Agassiz. Test depressed, highly ornamented. Ambulacra with well-developed pore-bearing zones, undulating, and each plate with threeto seven pairs of pores in an arc. Pairs of pores in two rows atthe apex and crowded at the mouth opening. Interambulacra withtwo or more vertical rows of primary tubercles which are imper-forate and crenulate, like those of the ambulacra. Apical systemencroaching upon the posterior interambulacrum. Jurassic-Recent. 23. C, volanum Cragin. Comanchic. Small, similar in form to Pseudodiadema texanum; pore beltssinuous; ambulacral area with two vertical rows of primary tuber-cles, surrounded by narrow strings of granules in polygons; inter-ambulacral area with two rows of large primary tubercles, with arow of smaller ones on either side of the area. Upper Washita of Texas. Order HOLECTYPOIDA Duncan. XVI. HoLECTYPUS Desor. Ambulacra narrow, straight, widest at the ambitus; some of theplates compound. Interambulacra with rather large plates and. Fig. 1922. Holectypus planalus. a, lower, b, upper, c, posterior view; d,ambulacral plates much enlarged ; <?, apical system enlarged. (After Clark.) 590 NORTH AMERICAN INDEX FOSSILS. many rows of tubercles. Mouth opening ten-angled. Anal open-ing large, pear-shaped, situated between the mouth and the pos-terior edge of the test. Apical disk small, central. Jurassic-Cretacic. 24. H. planatus Roemer. (Fig. 1922.) , subconical, flattened on under surface. Ambulacra with six irregular rows of tubercles. Pores in single pairs. Inter-ambulacra with numerous narrow plates, each plate with a nearlyhorizontal row of small tubercles. Widely distributed in the Fredericksburg and Washita of Texas. 25. H. charltoni Cragin. than preceding and less elevated; quinquelaterally rotund; apex tending to rise slightly from a somewhat flattened summit-region ; periproct two or three times smaller in proportion than inpr
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