REPORT ON THE RADIOL ARIA. 1019 



distinguish the large pores of the ventral plate as facial pores (upper orbital, middle 

 nasal, and lower maxillary pores), and the opposite large pores of the dorsal plate as 

 occipital pores (upper epoccipital, middle mesoccipital, and lower suboccipital pores). A 

 closer comparison of these pores, and of the separating bars in the numerous S p y r o i d e a, 

 may show a regularity of development similar to that offered by the homologies of the 

 parts in the skeleton of the Echinodermata, or of the bones in the skull of the 

 Vertebrata. 



The two convex lateral plates of the cephalis, or the right and left sides, both 

 symmetrically equal, do not exhibit the same regularity in the shape, number, and 

 disposition of the pores that the four other sides do. Usually theii* pores are much 

 smaller and more numerous. In very few forms only a distinct frontal ring is visible 

 (corresponding to that of the Coronida), and in this case the lateral pores are sometimes 

 disposed symmetrically on both sides of this ring. These Spyroidea may have 

 arisen directly from corresponding forms of Coronida. 



The sagittal ring (or the primary vertical ring, inherited from the Stephanida and 

 Semantida, shows in the Spyroidea great variety in its form and in its relation 

 to the cephalis. These variations are far greater than Biitschli (1882, loc. cit.) 

 supposed. We may distinguish the following six principal cases : — A. The ring lies 

 completely in the wall of the cephalis, and causes a deep sagittal constriction in it. In 

 this case the lower part or basal segment of the ring separates the basal pores into pairs ; 

 its anterior part or ventral rod the facial pores ; its upper part or coryphal rod the apical 

 pores ; and its posterior part or dorsal rod the occipital pores. No part of the ring is 

 free in the shell-cavity. B. The greater part of the ring lies enclosed in the shell- 

 waU ; only its dorsal rod arises free in the shell-cavity and ascends vertically or 

 obliquely to the apex, where it is usually prolonged into the apical horn. This seems 

 to be the most common case. C. The coryphal and the basal part of the ring lie 

 enclosed in the shell-wall, with its dorsal and ventral rod fi'ee in the shell-cavity ; the 

 dorsal rod ascends vertically to the apex, the ventral rod obliquely upwards to the 

 upper part of the facial plate (the nasal or orbital region). This case, regarded by 

 Biitschli as the usual one, is far less common than he supposed. D. The greater part 

 of the ring lies free in the shell-cavity, its basal rod only is enclosed in the shell-wall, 

 and separates the right from the left group of the cortinar pores. This case seems to 

 be rarer than the preceding. E. The whole ring lies free in the shell-cavity, and is 

 connected with the sagittal constriction of the shell -wall by numerous short radial beams. 

 The distance of the sheU-waH from the enclosed ring is usually least on the basal 

 rod, and greatest on the dorsal rod. F. The whole ring lies free in the sheU-cavity (as 

 in the preceding case), and is connected by numerous short radial beams with a secondary 

 larger, concentric sagittal ring, which is developed in the longitudinal constriction of 

 the shell-wall. 



