THE TETRABRANCHIATA. 450 



secretion into the ovarian sac. The vas deferens similarly 

 takes its origin, not from the sac of the testis but from a 

 smaller chamber communicating therewith. The commence- 

 ment of the vas deferens is enlarged and glandular. Nothing 

 is known of the development of the Tetrabranchiata. 



The only existing representatives of the Tetrabranchiata 

 are the different varieties of " pearly nautilus " (Nautilus 

 pompilius), which are found in the southern seas, living at 

 the bottom at a considerable depth. The genus is one of the 

 oldest in existence, since it is traceable through the whole 

 series of fossiliferous rocks as far back as the Silurian 

 epoch. 



Along with it, in the Palaeozoic formations, occur numer- 

 ous closely-allied forms, which differ from Nautilus mainly in 

 the different curvature (Lituites, Gyroceras, Trochoceras) or 

 straightness (Orthoceras, Gomphoceras) of the shell, and in 

 the varying position, proportions, and degree of calcification 

 of the siphuncle. 



In the middle of the Palaeozoic strata (Devonian), Tetra- 

 branchs (Ammonitidce) appear, in which the margins of the 

 septa are strongly bent, whence their edges appear as zigzag 

 transverse lines, folded into lobes and saddles, when the outer 

 layer of the shell is worn away (Goniatites, Ceratites); and, 

 in the Mesozoic epoch, the lobes and saddles become extreme- 

 ly complicated, while the shells may be straight, simply 

 curved, or bent, or turbinated (Ammonites, Bacidites, Turri- 

 lites). The Ammonitidce are extraordinarily numerous in 

 the Mesozoic epoch, but no trace of them has been found in 

 tertiary or quaternary formations. 



Associated with Ammonites, and not unfrequently lodged 

 in the terminal chamber of the shell, are the so-called Aptychi. 

 These are plates of a shelly substance, three-sided, with 

 rounded-off angles, and applied together by their straightest 

 edges so as to resemble bivalve shells. They consist of two 

 layers, an inner and an outer, of which the inner presents 

 lines of growth, concentric with the angle of each plate which 

 is situated on that side of its broad end which is applied to 

 its felknv. The outer layer is composed of many laminae, and 

 is traversed by pores. Its free surface frequently presents 

 longitudinal ridges. The heart-shaped plates, undivided by 

 a suture, wilich are found in some Goniatites and Ammonites, 

 are termed Anaptychi. 



The Aptychi, when undisturbed, occupy the middle of the 

 posterior wall of the terminal chamber of the Ammonite, and 



