Hall.] 248 [May. 



the same appendages in part, or in a partially developed condition ; the 

 distinct tube only being wanting. But had we the means of examin- 

 ing the internal characters of the ventral valves of all the species of 

 Spirifers, we should probably find graduations from the solid filling 

 of the rostral cavity, with a greater or less extension of shelly matter 

 in the form of a septum in the fissure occupying a narrow space in 

 its apex, till we reach the development observed in S. alta and S. 

 textus. 



To a considerable extent, we are able to prove this supposition. 

 In the separated valves of Spin/era radiata of the Clinton and 

 Niagara groups, there is a filling of the rostral cavity, and a thicken- 

 ing of the dental lamellae which are extended for half the length of 

 the valve. The narrow median crest of the muscular impression is 

 expanded and thickened towards the apex of the valve, but there is 

 no evidence of a septum. In Spirifera niagarensis there is a filling 

 of the rostral cavity and a thickening of the dental lamellae, which 

 are distinctly separate ; but no appearance of a septum : there is a 

 low median crest in the muscular impression. 



In the Lower Helderberg group, the Spirifera macropleura has 

 the rostral cavity filled to a greater or less extent, and the dental 

 plates extremely thickened at their bases ; while there is an obscure 

 low median crest in the muscular impression, which sometimes be- 

 comes thickened and expanded above; and occasionally there is a 

 little filling of the upper part of the fissure, resembling the incipient 

 stages of growth of the septum. In Spirifera cycloptera the rostral 

 cavity is more or less filled and solid, with diverging dental lamellae; 

 while the crest dividing the muscular impression is very unequally 

 developed, and there is no appearance of a septum. In Spirifera 

 perlamellosa the rostral cavity is filled to a greater or less degree, the 

 dental lamellae are short and strong, and there is a sharp elevated 

 median crest or septum in the muscular impression ; but there is no 

 indication of the transverse septum. In Spirifera octocostata the 

 dental lamellse are thickened, and there is a vertical septum reaching 

 from the apex of the rostral cavity to the base of the muscular im- 

 pression. In Spirifera marylandica of the Oriskany sandstone, 

 there is sometimes a partial filling of the rostral cavity, with a reversed 

 conical crest in the upper part of the muscular area. Spirifera 

 arenosa has strong dental lamellae ; a filling of the rostral cavity in 

 old shells ; a low crest, dividing the muscular impression, which be- 

 comes thickened in its upper part ; a distinct pseudo-deltidial cover- 

 ing of the fissure, but no transverse septum. 



