18 JURASSIC EAUNA. 



umbilicus remains. The ribs, which are very fine and sharp, begin at the umbilica 

 edge, and divide into two or three branches at about the middle of the height. 

 These secondary ribs are of equal strength, are equidistant and pass over the 

 siphonal side. 



Jjocality and stratigrapTiical position, — Mazar Drik ; Polyphemu8-\\mes,toTae. 



Remarks. — This species is rather rare at Mazar Drik, and I doubt if I should 

 ever have been able to identify it with the above species, had I not had Profes- 

 sor Waagen's original for comparison. This species shows, as Professor Waagen 

 correctly remarks, a great resemblance to Macrocephalites transiens, Waagen, and 

 it is rather difficult to distinguish even well-preserved specimens of the two forms 

 from each other. According to Professor Waagen the chief distinguishing features 

 of Macrocephalites opis consist in the narrow umbilicus and the smaller curve 

 of the ribs towards the front on the siphonal side. It is quite clear that such delicate 

 and minute distinctions can hardly he noticed on a specimen that has been in any 

 way deformed by pressure. If I were solely to rely upon these two distinctive 

 features, I should have been unable to identify the specimen described above, the ribs 

 having been unquestionably squeezed out of position ; it is, however, quite certain 

 that the umbilicus is very narrow — narrower in fact than it would be in a specimen 

 of Macrocephalites transiens, Waagen, of the same size. 



On comparing it with the original of Waagen's fig. 2, 1 found that the character 

 of the ribs agrees so well with that form that they must be considered as identical. 



It must, however, be noticed that the character of the ribs of this species is 

 rather variable. One of Professor Waagen's originals, which is of much smaller 

 dimensions than his fig. 2, but larger than fig. 3, exhibits much more inflated 

 whorls and stronger and less regular ribs than tig. 2. It must further be men- 

 tioned that his fig. 2 has been considerably restored in the right-hand upper portion, 

 and I am of the opinion that the cross-section of the last whorl is not quite in 

 accordance with the original which is a little more compressed at the umbilical 

 edge than is shown in the figure. 



2. Sub-family : PBRISPHINCTINJS, Steinm. 

 Genus : PERISPHINCTES, Waagen. 

 Perisphinctes balinensis, Waagen (non Neumayr). PI. XIII, fig. 1. 



1875. Perisphinctes balinensis, Waagen, Palseontologia Indiea, Jurassic Fauna of Kutoh, VoL I p 163 

 PI. XLV, fig. 2a, b. ' f > 



Dimensions. I, jj_ 



Diameter of the shell 67 yj 



„ of the umbilicus ,29 29 



Height of the last whorl from the nmbilical suture .... 21 23 



» » „ from the preceding whorl .... 17 20 



Thickness of the last whorl . Ig ^8 



The shell consists of more than six flat, low whorls, which very gradually 

 increase in height, and overlap each other only to a small extent. The umbilicus 

 is therefore wide and the inner whorls are well exposed. The umbilical wall is very 



