AEGOCERAS ACUTICOSTATUM. 371 



area, the increasing prominence of the tubercles, and the ribs proceeding from them 

 becoming split up into two or three divisions, thus covering the area with lines of transverse 

 sculpture. In PI. XXXVI, fig. 1, we have another specimen of still larger size collected in 

 Robin Hood's Bay ; here we observe a closer approximation of the ribs as they approach 

 the aperture, and their comparative smallness when contrasted with those on the 

 penultimate whorl. In fig 2 the thick obtuse ribs seen on the front view of this fossil, 

 form a remarkable contrast to the narrow transverse ribs which regularly cover the wide 

 convex siphonal area so well delineated in fig. 3 of the same plate. 



The lobe-line is extremely complicated (fig. 4). The siphonal lobe is as long and 

 wide as the principal lateral, and ornamented on each side with extended branches, of 

 which two are transverse. The siphonal saddle as wide as the principal lateral lobe 

 terminates in three foliations, of these the central is the largest, with a trifid foliole ; 

 the principal lateral lobe is highly ornate, with two large external and two small 

 internal branches, and a long terminal ramified branch. The lateral saddle ends in 

 two unequal foliations. The lateral lobe is small and divides into three digitations, and 

 the auxiliary lobes are much smaller and end in single digits. 



Affinities and Differences. — Aegoceras heterogenum very much resembles Aegoceras 

 Henleyi in having a youthful form of shell entirely different from its adult condition, both 

 having passed <;hrough a remarkable transformation in the figure and capacity of the 

 body-chamber about the sixth volution of their growth. I have compared the two forms 

 very closely with each other, and my specimens lead me to the conclusion that Aeg. latce- 

 costa, the young form of Aeg. Henleyi, is distinct from Aeg. capricornus, the young con- 

 dition of Aeg. heterogenum, and that the adult states of both species are evidently distinct 

 from each other. 



Locality and Stratigrapliical Position. — This Ammonite is very rare on the Yorkshire 

 coast ; it is occasionally found at HuntclifF, and in Robin Hood's Bay in beds referred to 

 the zone of Aegoceras capricornus. I have one example from a ferruginous shaly band, 

 and another, figured in PI. XXXVI, which I collected from the Marlstone of Yorkshire. 



Aegoceras acuticostatum, Wright, nov. sp. PI. XXXV, figs. 1 — 3, 7. 



Diagnosis. — Shell discoidal, depressed ; whorls rounded and slightly involute ; sides 

 ornamented with twenty-four sharp waved ribs, which pass found the margin and 

 disappear from the middle of the siphonal area ; umbilicus wide ; inner whorls all 

 exposed ; aperture oblong. 



Dimensions.- — Transverse diameter 85 millimetres ; width of umbilicus 38 millimetres ; 

 height of aperture 30 millimetres ; transverse diameter 25 millimetres. 



Description. — This is a very distinct form, which belonged to the cabinet of ray late 

 friend, Mr. J. Leckenby, F.G.S., and was collected from the Aegoceras Jamesoni beds of 



