CETOTHERES FROM THE MIOCENE CHOPTANK FORMATION 



Pedicles of neural arch of third to sixth cervicals short, 

 widened: neural canal low, unusually wide; roof of 

 neural canal slightly arched, almost horizontal. Neural 

 spines of first to fourth dorsals short, nearly vertical, 

 not noticeably slanting backward and their height equiv- 

 alent to less than half vertical diameter of correspond- 

 ing vertebra, in contrast to somewhat longer backward 

 slanting neural spines of ninth to twelfth dorsals whose 

 height is equivalent to or sliglitly more than half vertical 

 diameter of corresponding vertebra. Neural canals of 

 fii'st to eighth dorsals low, very wide; roof of neural 

 canal not arched, nearly horizontal ; diapophyses stout, 

 broad, dorsoventrally compressed. Elongated, backward 

 slanting neural spines present on first to ninth lumbars; 

 metapophyses ratlier slender. Transverse process of 

 first to fourth caudals short and broad, not widened 

 distally. Scapula wide, acromion broad, coracoid process 

 short and stout, and prescapular fossa very narrow. 

 Length of humerus about 67 percent of length of radius. 

 Posterior process of periotic elongated, ratlier slender, 

 increasing in diameter distally; anterior process com- 

 pressed from side to side, deeply concave internally and 

 convex externally ; posterior profile from a ventral view 

 deeply indented between pars cochlearis and posterior 

 process. 



Thinocetus arthritus, new species 



Type-specimen. — USNM 23794. Posterior portion 

 of cranium, incomjilete ; right and left tympanic bullae, 

 right and left periotics, left mandible lacking section 

 near anterior end, atlas, axis and 3 cervicals, 12 dorsals, 

 12 lumbars, 12 caudals, 3 chevrons, right scapula com- 

 plete, left scapula lacking anterovertebral angle, right 

 and left humerus, right and left ulna, right radius, 7 

 carpals, 7 metacarpals, 3 phalanges, 22 ribs. Collectors, 

 Albert C. Myrick, Jr., and Charles F. Buddenhagen, 

 September 21-25, 1966. 



Horizon and locality. — Low cliff east of Mud Cliffs 

 on south shore of Potomac River (7/10 mile west of 

 Haulover Point on Haulover Inlet), 180 feet west of 

 Md.-Va. Boundary Monument 2-4, Westmoreland Coun- 

 ty, Virginia. In greenish sandy silt in transitional layer 

 three feet ten inches below zone 17 shell bed containing 

 Isognommen^ Pecten jmidisanhi.'i, Perfen marylandica, 

 Saxolutina D.P., Crmsatella turgidula, and Ecphora 

 quadricostata. Choptank formation, middle Miocene. 



Referred specimen. — One, as follows: I'SNM 

 22961; left mandible lacking portion of ramus behind 

 coronoid process, axis, third cervical, lumbar, and a 

 terminal caudal, coll. Thomas E. Stokel, November 

 1962, in zone 17 at Cape St. Marys, east of Hollywood 



and on Patuxent River, St. Marys County, Md. Chop- 

 tank formation, middle Miocene. 



SKULL 



Prior to excavation the entire rostrum and all of the 

 braincase in front of the lateral protuberances of the 

 basioccipital had been broken off and destroyed when 

 the skull was exposed on the face of the cliff either dur- 

 ing erosion by the Potomac River at flood time or sub- 

 sequent slumping. 



Tlie vertical partition between the choanae, which 

 seemingly diminished rapidly posteriorly, is continued 

 backward to within a few millimeters of the end of the 

 horizontally flattened portion of the vomer whicli con- 

 ceals the transverse contact between the basisphenoid 

 and the basioccipital. No remnants of either pterygoid 

 remain attached to this portion of the basicranium (pi. 

 1, fig. 1). The ventral surface of the large descending 

 knob-like protuberance on each side of the basioccipital 

 is somewhat eroded. This protuberance does not, liow- 

 ever, contribute the inner wall of the notch or incisure 

 for the jugular leash, although it is continuous poste- 

 i-iorly with the thin descending external border of the 

 basioccipital which does limit this side of the posterior 

 lacerated foramen. The external wall of this notch is 

 formed by the exoccipital. The inner faces of these 

 lateral protuberances are separated by an interval of 

 50 mm. 



The greatest width of the tympanoperiotic recess did 

 not exceed 50 or 60 nun. In front of each tympanoperi- 

 otic recess the pterygoid bone that constitutes most of 

 the osseous walls of the pterygoid fossa has been de- 

 stroyed on this basicranium. 



The right postglenoid process is broken off at the 

 base and the zygomatic process is also missing. 



The left squamosal lacks the anterior face of the 

 postglenoid process and all of the zygomatic process. 

 This left postglenoid process is deflected obliquely back- 

 ward and its thin anteroposteriorly compressed extrem- 

 ity is extended at least 50 mm. below the level of tlie 

 lateral protuberances of the basioccipital. Viewed from 

 beliind the acute profile of the ventrally prolonged 

 postglenoid process results from an oblique external 

 truncation and a shallowly concave internal curvature. 

 Between the posterior process of the periotic and the 

 base of the concavely curved posterior face of the post- 

 glenoid process is the deep transverse channel for the 

 external auditory meatus; this channel is narrower ex- 

 tenialiy tliaii internally. A broad shallow concavity on 

 the ventral surface of tlie right squatnosal external to 

 the pterygoid fossa extends backward to its sharp-edged 



