STEWART: OSTEOLOGY OF ANOGJMIUS POLViMICRODUS. 121 



The opercular bones are thin and scale-like, and are all in place. 

 They are covered with fine striae and are so thin at the edges that 

 the different directions that these striae take are the only means of 

 locating the boundary lines between some of the different elements. 

 Tlve preoperculum is small, narrow above and extended well for- 

 ward below. The operculum is large and is extended for some 

 distance above its articulation with^ the hyomandibular. The 

 inter- and sub-operculars extend backward as far as the operculum. 



In one specimen there are seventy-two vertebrpe exposed to view, 

 and there are probably eight or ten more hidden by the opercular 

 bones just back of the skull, making in all about eighty vertebrae 

 in the complete column. The centrae are all rather short and do 

 not present the lateral grooves found in the Saurodontidi? and 

 Ichthyodcitidtc, but are striated and somewhat resemble the verte- 

 bra; of the genus PacliyrJiizodiis Agassiz in this respect. Just in 

 front of the caudal fin the vertebra are crowded together, and 

 those supporting the fin are very much so. The last vertebra is 

 succeeded by a fan-shaped expansion or urostyle, which presents a 

 well-marked ridge on the side exposed. The neural arches fit into 

 pits at their bases and in the caudal region they have lamina of 

 bone projecting down the sides forming yoke-like articulations with 

 the centrae. The arches are expanded at their bases and are larg- 

 est in the anterior region, where they are directed backward but 

 slightly, while in the region of the fiftieth posterior vertebra the 

 arches are very slender and directed strongly backward. The 

 manner of attachment of the haemapophyses can not be made out 

 certaint}', although they were probably inserted in pits as are 

 the neurapophyses. The ribs are long and slender and slightly 

 expanded at the proximal extremities. They seem to be inserted 

 in pits on the sides of the centra;. The caudal fin is homeocercal 

 and is composed of numerous rays which unite with the centrae by 

 means of the yoke-like articulations mentioned above. Tovv'ard 

 the distal extremity the rays split up into many thread-like fila- 

 ments, which makes this portion very difficult to collect in a perfect 

 condition. The body is covered with medium-sized elliptical 

 cycloid scales. The total length of this fish was about 1.82 m. 

 There is not enough of the fin-remains present to determine their 

 character. 



Lawrence, Kansas, May i§, i8g<^. 



