M 



YORKSHIRE TYPE AMMONITES— II August 



The suture-line, which is of a simple pattern, shows short and broad 

 lobes and shallow ES ; L 1 is particularly broad, and S 1 is somewhat 



contracted. • 



This genus appears to connect the Liparoceratidae and Ecmocera- 

 tidae, showing that they have a common origin in a capricorn-like form 

 with simple suture-line. 



The present genus has a simpler suture-line than Bifericeras, and 

 instead of passing rapidly from smooth to bituberculate it shows a very 

 prolonged costate stage. From the absence of bituberculation and the 

 suspicion of carination like Echioceras, it may be adjudged a primitive 

 series of Echioceratidae. But if the carination be accidental or transient, 

 and if carinate descendants be not produced, then it should be reckoned 

 as a primitive Liparoceratid. 



Genus, DEFOSSICERAS, nov. 



Type, Am. defossus, Bean-Simpson (see No. 76). 



Serpenticones similar to Oistoceras, with similar tongue-like costae 

 on the venter, but these are joined to a rudimentary carina. 



The ribs on the inner whorls are coarse and distant, instead of small 

 and approximate ; later ribs are slightly curved on the lateral area 

 instead of straight. The whorls themselves are squared instead of 

 roundish, thickest about the inner margin, which is fairly defined. There 

 is a feeble tuberculate stage, but no sign of bituberculation and swollen 

 whorls, as in Oistoceras. 



The suture-line, though simple, differs much from that of Gagaticeras 

 in having a small L 1 close to outer edge of whorl, just inside genicular, 

 and a broad S 1 which is carried well forward. 



The confusion of Am. defossus with Am. maculatus (Simpson, 1843, 

 15), Blake's figuring of Oistoceras omissum as Aigoceras defossum (1876, 

 viii, 9, p. 282), his remarks as to the similarity to " Amaltheus spinatus " 

 (p. 282), and the likeness of Am. defossus to Beaniceras, species of which 

 have been confused with Am. capricornus (maculatus), all indicate that 

 this genus shows a curious combination of characters : it begins like 

 a Liparoceratid and becomes like an Amaltheid — Paltopleuroceras. 



The beginning in Am. defossus is similar to that of Beaniceras luridum 

 (No. 73) ; The venter with linguiform costae is like Oistoceras ; the 

 breaking up of the costae on the venter is similarly seen in Oistoceras 

 and Beaniceras. The carination of the periphery and other small features 

 distinguish it from these genera, but liken it to Paltopleuroceras, which, 

 however, shows much more peripheral projection. 



The likeness of Am. defossus, with its simple and distinctly Arietan 

 suture-line to species of Agassiceras, is even more remarkable ; but this 

 does not seem to have attracted attention. Yet that is the genus which 

 casual examination would suggest for Am. defossus. Differences of 

 ornament appear to be trivial : they are only important as indices of 

 development. There is a stout costate instead of a smooth larval stage, 

 there is a slight lateral curvature of ribs, and linguiform flattened costae 

 on the venter connected by a slight keel, not of continuous strength, 

 as distinct from the median continuous keel of Agassiceras, which is 

 joined by single ribs. The general likeness is an example of cyclical 

 homoeomorphv : Agassiceras is anagenetic— smooth to tuberculate ; 

 Uejosstceras is catagenetic, presumably past the acme of the. tuberculate 

 stage. 



