. Bulletins of American paleontology. PERTURE Cerithium miiscarumText-figure 51A, B AMOCO Taxon #.—2091 Diagnosis.—Narrow shells about 25 mm high. Pleu-ral angle (the angle formed by lines drawn from theapex of the shell, downward to points tangent to thecurve of the last whorl on either side of the shell) 25°-28°. Ornament typically of strong axial (vertical) ribsthat are not aligned from whorl to whorl. Fine spiralthreads cross the ribs, and knobs tend to develop attheir intersections. This form intergrades with Cerithium atratum (Text-fig. 51C, D), described below, through shells in whichth
. Bulletins of American paleontology. PERTURE Cerithium miiscarumText-figure 51A, B AMOCO Taxon #.—2091 Diagnosis.—Narrow shells about 25 mm high. Pleu-ral angle (the angle formed by lines drawn from theapex of the shell, downward to points tangent to thecurve of the last whorl on either side of the shell) 25°-28°. Ornament typically of strong axial (vertical) ribsthat are not aligned from whorl to whorl. Fine spiralthreads cross the ribs, and knobs tend to develop attheir intersections. This form intergrades with Cerithium atratum (Text-fig. 51C, D), described below, through shells in whichthe spiral threads become stronger and the axial ribsless pronounced. Comparison.—Cerithium atratum has dominantlyspiral ornamentation, becoming spiral rows of smallknobs on the adult whorls. Remarks.—The best way to separate the severalforms of Cerithium is to check the pleural angle it is narrow (25-28°), the shell is C muscarum. Ifthe pleural angle is larger, separation depends upon theornamentation. Bulletin 370. B bA ^^^ 1 1
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