m. 1.] WHITE ON JURA-TRIAS FOSSILS OF IDAHO. Ill 



Other in their surface-markings, and are evidently all strictly congeneric; 

 but it is probable, as Meek has suggested in relation to A. idahoensis, 

 that neither of them will be found to conform strictly with the typical 

 examples of Aviculopecten. Their reference to that genus is therefore 

 to be regarded as provisional only in this instance. 



CEPHALOPODA. 



Five species of Cephalopods, and perhaps six, are represented among 

 the collections brought in from localities ISTo. 1 and No. 2, three of which 

 belong to the genus here proposed by Professor Hyatt, and the others 

 probably to Arcestes. The following are Professor Hyatt's diagnostic 

 remarks upon the proposed new genus. 



Genus Meekoceras* Hyatt. 



"These species, so far as they go, are unlike the Ceratites of any 

 foreign locality, but have more resemblance to the Muschelkalk than to 

 the St. Cassian or Hallstadt faunas. They possess in common one char- 

 acteristic which separates every species from the typical forms of Eu- 

 ropean Ceratites. There are but three distinct lateral cells and two 

 lateral lobes besides the finer auxiliary lobes and cells. This occurs in 

 the most involute species — "C" [M. gracilitatis], as well as in the least 

 involute — "A" [M. a/planatum]. This characteristic would be of no small 

 value in any groui>, but in this one it is unusnally constant in spite of 

 the great differences of form and the variations in breadth of the sides 

 of the whorls between the different species. The typical Ceratites, the 

 C. nodosus and C. semipartitus, have at least four distinct lateral cells 

 and lobes besides the auxiliary ones, and the distinction is slight be- 

 tween the two series. In this genus, on the contrary, the auxiliary series, 

 when present, is not divided from the third lateral cell by a distinct lobe 

 as in Ceratites, and the aspect of the third lateral cell is often like that 

 of a Goniatites. The auxiliary series is of course not present in the less 

 involute and narrower sided forms, such as "A" [M. aplanatum], and 

 Amm. parens, Amm. hoydianus, and Amoii. ottonis, as figured by von 

 Buch; and Ceratites carhonarius Waagen. These and the Goniatites 

 IcBvidorsatus Gabb (which I have in Meek's report on the Paleontology 

 of the Geol. Expl. 40th Parallel erroneously referred to Clydonites) are 

 quite distinct, but the adult sutures of the latter are not known and its 

 Ijosition is therefore uncertain. 



"The compressed whorls of all the species is of course a characteristic 

 which is obvious when they are contrasted with typical Ceratites, as is 

 also the absence, or merely transient appearance, of heavy nodes and 

 ribs, except perhaps in the least involute species, if Icevidorsatus be 

 found to belong to this genus. The young shells appear to be quite dis- 

 tinct from the young shells of the true Ceratites so far as these have 



* "Dedicated to tlie memory of my Mend F. B. Meek as some slight testimony of 

 my respect for liis works and regret for his loss." — (A. H.) 



