CEPHALOPODA. 245 



to the depth of the chambers. Specimens in which the siphuncle has 

 become exposed by weathering, show that it was obscurely lobed and longi- 

 tudinally ribbed. 



The test was probably thin. No portions of it are preserved in any of 

 the specimens observed. The surface, as shown by the internal casts of 

 several individuals, was marked by lamellose, transverse stride. 



The appearances of the internal mould of this species are very character- 

 istic. The exsolute, or lax condition of the chambers, together with their 

 concave margins — a uniform condition of the compressed specimens — is a 

 distinguishing feature. The organic deposit is shown in all casts of the 

 interior, and is peculiar to the species. 



The largest fragment referred to this species has a length of 200 mm. 

 Other fragments have been found belonging to larger individuals. The 

 maximum length was probably not more than 400 mm. 



The organic deposit on the septa, around the siphuncle, and on the ventral 

 walls of the air-chambers, presents many different aspects, and has been 

 observed in numerous individuals. It increases in amount and complexity 

 of detail, from the chamber of habitation to the apex. In some specimens 

 the siphuncle has been absorbed or obliterated by the deposit. Other speci- 

 mens show a deposit formed on the interior of the siphuncular tube. The 

 filling up of the chambers is not, however, carried to such a remarkable 

 extent as in 0. oppletum, but is much more marked than in any of the pre- 

 ceding species. The deposit on the septa varies in different portions of the 

 tube, and on different sides. Sometimes the amount is very marked on one 

 ride (see fig. 3, pi. 78 B), and less prominent or nearly absent on the other. 

 The appearance produced is generally a pitting or erosion of the material 

 filling the chambers. In well-preserved specimens, it is shown that these 

 pits are the casts of the original deposit, which consists of closely arranged, 

 irregular, mammillary aggregations of calcareous matter. The globules com- 

 posing these masses are usually very small, occasionally reaching a diameter 

 of two mm. The appearance around the siphuncle is, at first, that of a 

 simple areola; the amount of deposit increasing till it becomes thickened, and 



