CRUSTACEA— TRILOBITA. 255 



5*. Glabella about one half the length of the cephalon. 



XXIII. Agraulos. 



d. Glabella with a large central and one to three side lobes .' 55. 



55. Pleura grooved, also segments of pygidium dd. 



dd. Cephalon large, about one third the length of the whole animal. .5t. 



5f , Pygidium short, axis usually less than 14 segments 6*. 



6*. Glabella extending practically to anterior edge of ceph- 

 alon 6'\ 



6'^. Glabella with subparallel sides XLVI. Proetus. 



(i". Glabella with concave sides XLIX. Griffi,thides 



6*. Glabella about three fourths the length of the cephalon. 



XLVII. Cyphaspis. 

 5f . Pygidium elongate, its axis usually with more than 14 seg- 

 ments XLVIII. Phillipsia. 



dd. Cephalon broad, short, about one fourth the length of the whole 



animal LI-LVI. LichadidcB. 



55. Pleura ridged ee. 



ee. Pygidium with spinose margin 6f. 



6f. Occipital ring with spines LVII, Acidaspis. 



6f . Occipital ring smooth or with anterior tubules. 



LVIII. OdontopleurU' 

 ee. Pygidium with spineless margin LIX. Glaphurus. 



II. Glabella of cephalon and usually axis of pygidium not distinctly defined 3. 



3. Body not trilobed except very slightly on cephalon XLIII. Bumastus. 



3. Body distinctly trilobed e. 



e. Axis broad, wider than pleura 66. 



66. Eyes placed anterior to the middle of the cephalon XLV. Nileus. 



66. Eyes placed medially or posterior to middle XLI. hoteles. 



e. Axis comparatively narrow 77. 



77. Free cheeks long, terete. Axis of pygidium deeply defined. 



XLIV. Thaleops. 



77. Free cheeks short, flat.. if. 



ff. Axis of pygidium inconspicuous XLI I. Illcenus. 



ff. Axis of pygidium more or less conspicuous. 



XXXIX. Asaphellus. 



C. Free cheeks showing above ; genal angles borne by fixed cheeks. Compound eyes 



present. Proparia III. 



III. Glabella narrowing anteriorly 4. 



4. Body indistinctly trilobed. Pygidium elongate, triangular. 



LXII. Ho77ialonotus . 

 4. Body strongly trilobed. Pygidium comparatively short, semicircular. 



LXI. Calymene 



4. Body unknown LXIV. Pseudosphoerexochus. 



III. Glabella ovoid or globular LXVI. Sphcerexochus. 



III. Glabella enlarging anteriorly (rarely subquadrate) 5. 



5. Glabellar furrows nearly or completely absent f 



f. Eyes very large LXVII. Phacops. 



f. Eyes small LX. Encrinurus. 



5. Glabellar furrows conspicuous g. 



g. Margin of cephalon bordered by a distinct rim LXV. Pliomera. 



g. Margin of cephalon without rim 88. 



