CRETACEOUS FOSSILS. 139 



has thrown light on the subject, and proves that the closely costato portions figured 

 on PL 16, belong together as the spiral, succeeded by a long subquadrate arm 

 without ribs, the " outer whorl" with the large ribs being a part of the recurved 

 portion of a true Ancyloceras. The species is enormous, but variable in size. The 

 specimen figured was about thirty inches in length, while one very perfect speci- 

 men in the cabinet of Mr. Voy, now deposited in the museum of the College of 

 California, is but seventeen inches long. 



? A. LINEATUS, n. S. 

 PL 23, Pig. 18, a, b, c. 



This species is only known by two fragments of the straight 

 limb. The section is ovate, a little narrower on the ventral than 

 on the dorsal side; surface marked by numerous, very slightly 

 oblique linear ribs, with broad concave interspaces; each rib 

 bears on its dorso-lateral face a minute tubercle. 



Septum composed of a dorsal and ventral lobe, and two laterals 

 on each side, giving three saddles on each side. Dorsal lobe 

 divided a little below the middle, with two teeth over the siphun- 

 cle, each branch bearing two spurs on the outer side, one large 

 spur and one very small one on the body of the lobe. Dorsal 

 saddle broad, deeply divided into two bifurcate parts. Superior 

 lateral lobe broad, as long as the dorsal, bifurcate in the middle, 

 each branch again bifurcate, and with a large spur on its outer 

 side above the subdivision ; the body of the lobe bears a single 

 large spur above the fork. Lateral saddle like the dorsal. In- 

 ferior lateral lobe like the superior lateral, but a little smaller. 

 Ventral saddle like the others, but a little smaller. Ventral lobe 

 shorter than the others, trifurcate, the branches dentate, two 

 small spurs above the branches. 



This septum is unlike any other in California, unless it be that 

 of ? Hamites quadratics, which it resembles in the number and 

 fundamental plan of structure of the lobes. In that species the 

 saddles are proportionately very much larger, and the lobes are 

 more compact; the degree of complexity could be accounted for 

 by the difference in size. The two shells differ materially in form. 



