86 MEMOIRS OF THE STATE MUSEUM 



with a corresponding fold in the ventral, beginning soon after the initial 

 stage of the shell has been passed, and disappearing before the shell is 

 half-grown. Those forms presenting this feature to a marked degree 

 are Rhynchotreta cuneata, Rhynchonella whitii, R. indianetisis, R. ncglecta, 

 R. acinus, Atrypa reticularis, and Retzia sobrina. In Rhynchotreta cuneata 

 and Atrypa reticularis (plate 4, figs. 12-15, ^"<^ plate 6, figs. 12-14), 

 we have shown the gradual inception of this sinus, its maximum develop- 

 ment and obsolescence, and, finally, its reversion into a fold which there- 

 after persists and usually increases in prominence in all the succeeding 

 stages of growth. The embryonal sinus is not present in Spirifer, 

 Anastrophia, Eichwaldia, Whitficldia, Meristina, Orthis, nor in the 

 Strophomenidae. Such of these as show a dorsal fold or ventral sinus, 

 have them developed early in the growth of the shell, and they usually 

 increase regularly to the time when the full size of the shell is attained. 



Beaks. The beak of the ventral valve in its earliest condition is 

 commonly erect, pointed outwards, and of a broad triangular form, while 

 that of the dorsal valve is small, not prominent, and lies in the longitudinal 

 axis of the shell. In all cases, the subsequent deepening of the valves 

 tends to incurve the beaks toward the cardinal area. The degree of in- 

 curvature varies greatly in the different species. Meristella rectirostra 

 shows a minimum, and Whitficldia maria or Atrypa reticularis, 2l maxi- 

 mum, and between these limits all intermediate conditions occur. The 

 usual degree of incurvature is presented in Spirifer radiatus, Retzia evax, 

 and the Rhynchonellidae. 



The outlines on plate 4, illustrating the profiles of the beaks in a 

 series of Rhynchotreta cuneata, represent an uncommon condition, for in 

 this species, the ventral beak, from its divergent initial position, gradually 

 approaches, and at maturity attains parallelism with the longitudinal axis of 

 the shell. It never becomes sufficiently incurved to conceal, to the slightest 

 degree, the deltidial area, while the initial dofsal beak becomes more 



