1020 THE VOYAGE OF H.M.S. CHALLENGED. 



Comparing these six principal cases, in respect of the relation of the primary sagittal 

 ring to the cephalis of the Spyroidea, we may suppose that they represent together 

 a continuous phylogenetical series, of which the first (A) is the original, and the last 

 (F) the latest case ; and that the true cause of the various changes is a successive 

 separation of the ring, which becomes more and more free and independent from the 

 lattice-plate of the cephalis. Since the intimate study of the structure of the cephalis in 

 many Spyroidea is very difficult, it requires further accurate observations (from all 

 six sides of the shell). There are also some other more difficult complications of its 

 structure, which cannot be solved without exact study and extended comparison. 



Of peculiar importance for the differentiation of the numerous genera of the 

 Spyroidea are the basal feet or the descending and diverging apophyses, which are 

 developed from the base of their cephalis. In general they exhibit the same typical shape 

 which we find in the Plectellaria as well as in the Cyrtoidea, and which we 

 regard therefore as an important common character of the majority of NASSELLARIA. We 

 may therefore distinguish here also the three primary, and the other ones as secondary 

 feet. The three primary feet, or the " cortinar feet," are the same that we found in the 

 Plagonida and Plectanida, in Cortina and Cortiniscus among the Stephoidea, and in 

 the majority of Cyrtoidea. They also reappear in the same characteristic disposition 

 and connection with the basal plate of the cephalis, in the majority of S p y r o i d e a. The 

 odd caudal foot (c) is the lower prolongation of the dorsal rod of the sagittal ring, which 

 is prolonged upwards into the apical horn. The two paired pectoral feet, however 

 (p'> P")> are the descending prolongations of the coracal rods (e) which separate the jugular 

 pores (i) from the cardinal pores (k) of the cortinar plate (compare PI. 95, figs. 2-6). 



The three cortinar feet are nearly equally developed in the majority of Spyroidea 

 (compare Pis. 84, 89). But often the odd caudal foot exhibits a different shape from 

 that of the two paired pectoral feet, and in many genera of the group it becomes more or 

 less rudimentary. It has quite disappeared in the subfamily Dipospyrida (PL 85) ; here 

 the two pectoral feet only are developed (often excessively), and are usually opposite in 

 the frontal plane. Such dipodal forms, very common in the Spyroidea, are never 

 found in the Cyrtoidea. 



The secondary feet, which we contrast with the three primary feet as later productions, 

 exhibit great variations in number and development. Very frequently three interradial 

 secondary feet become developed, alternating with the three primary cortinar or perradial 

 feet, and opposed to them in pairs (PL 95, figs. 7, 8). In these hexapodal Spyroidea 

 (or Hexaspyrida) an odd anterior or sternal foot (z) is opposite to the odd caudal foot, 

 and two paired posterior or tergal feet (t tl t n ) to the two pectoral feet. When the sternal 

 foot becomes rudimentary or lost, pentapodal forms arise (Pentaspyrida, PL 95, figs. 

 9-11), and when the two odd feet (caudal and sternal) disappear, tetrapodal forms 

 arise with two pairs of feet (Therospyrida, PL 89, figs. 5, 6). The latter differ from 



