Report of the State Geologist* 



185 



The inner margin of the binge in both valves frequently bears 

 a series of fine, interlocking dentieulatdons, making an arrange- 

 ment which contributes to the firmer union of the valves (Strop- 

 heodonta, Spirifek, etc.). Interlocking 

 plications about the lateral and anterior 

 marffins of the valves serve a similar end. 



3. 



Septa. 

 Vertical plates or septa divide the 

 interior cavity of the valves of some genera 

 in various ways. The teeth are frequently 

 supported by such plates, one on each 

 side of the delthyrium of the pedicle- 

 valve, and these are known as dental 

 plates. Such plates may rest upon the 

 inner surface of the valves or converge crura (c). (Davidson.) 

 and unite, forming a spoon-shaped process, or spondylium, which 

 is sometimes free, except at its posterior margin, but is usually 

 supported by a median septum (Clitambonites, Pentamerus, Cam- 

 arophoria, etc.). When the spondylium is not present in the 



Fig. 70.— Conchidium Knighti. 

 Longitudinal section, showing 

 the septa (s) and spondyli a 

 (a, 6) of both valves, and the 



Fig. 73. — Transverse section 

 Fig. 71.— Camarophorw Sch- of Conch id ium Knight, s, me- 



lotheimi. s, median septa; Fig. 72. — Sicberella Sieberi. dian septum of pedicle-valve: 

 crura; a.spondylium of pedicle- Transverse section showing s', septa of brachial valve; p, 

 valve ; 6, spondylium of brach- the spondylia supported by spondylium of pedicle- valve ; 

 ial valve. (Woodward.) median septa. b, crura, plates. 



pedicle-valve, a median septum often divides the muscular region 

 and itself becomes a surface of muscular attachment (Enteletes, 

 Spiriferina). There may also be one or more lateral septa on 

 each side of the median one, as in the genus Polytcechia (Plate 8, 

 figs. 16-20) ; this, however is of rare occurrence. 



37 



