10 HYATT ON THE TERTIARY SPECIES 



forms, since it is frequently the case that the adults of two species are closely similar when 

 the young are distinct. 



The four main varieties, or species found in the Pit Deposits, figs. 2, 9, 13, 17, pi. 9, are 

 not connected by hybrids, and are distinguished from each other by a practiced eye with 

 less difficulty than would seem possible to any casual observer, however well trained in 

 other fields.^ The remaining representative forms, such as 4-19 and 5-10, are also not 

 connected, and are quite distinct in the characteristics of their particular series. The First 

 Series, as has been stated, is as represented on pi. 9, divided' sufficiently for, general 

 consideration into three sub-series. The first sub-series leads up to distortus, fig. 28, a 

 variety of PI. costatus, fig. 26-27; the second to denudatus, a variety of PL minutus, fig. 

 21 ; the third to turhinatus, fig. 20, a variety of PI. triquetrus, fig. 19. The Second 

 Series is capped by PI. crescens, fig. 15, the Third Series by PI. supremus, var. turritiis, 

 fig. 11, and the Fourth Series by PI. trochiformis. These four series and sub-series 

 may be classified under three heads, according to the meaning of their ontological 

 characteristics. 



A, the purely progressive series, or those in which the special characteristics of the 

 series are developed more and more decidedly, and new ones added in each successive 

 species or form as in the Second, Third, and Fourth Series. 



B, the purely retrogressive, or those in which the differences observed in the Pit forms, 

 when compared with PI. levis, are not maintained in the progressive sense, but in which 

 disease interferes with progress, and leads to the production of distorted variations, as in 

 the second sub-series 21-24. 



c, the partly retrogressive and partly progressive series, in which the differences are 

 increased by the addition of certain minor peculiarities, but the forms become nevertheless 

 distorted by disease or decrease in size, as in the first and third sub-series. 



The resemblances of the forms 1, 8, 12, 16, pi. 9, are of course due to their close 

 affinity as varieties of PL levis, those of PL Steinheimensis, fig. 2, PL oxystomus, fig. 

 9, and PL minutus, fig. 18, belong, however, to a distinct species, and must be considered 

 representative forms. They belong, in two cases, to 2 and 9, to progressive series, and 

 18 to the partly progressive and partly retrogressive third sub-series. 



The representative forms, PL steM^mmsis, %• 3, PL %a^^!, fig. 13, PL minutus, fig. 21, 

 and PL SSj fig- 25, have even closer resemblances in outline than the original four 

 varieties of PL levis, and yet, on account of the absence of hybrids or intermediate forms, 

 are unquestionably more easily distinguished from one another, than these four original 

 varieties. The same observations apply to PL tenuis, fig. 4, and PL triquetrus, fig. 19 ; 

 PL discoideus, fig. 5, and PL supremus, fig. 10 ; PL "diJmuZi% fig- 6, PL supremus, var. 

 turritus, fig. 11, PL triquetrus, var. turhinatus, fig. 20. 



1 In justice to myself it ought to be remarked, that the fer by tracings, or otherwise. After all the figures of each 



method pursued in drawing the different figures on pi. 9, series had been made, they were arranged upon a dark 



was as follows: Each series was picked out without refer- brown tablet. I had but a slight suspicion even of the 



ence to the formations, merely to show the zoological rela- remarkable nature of the ontological relations here des- 



tions. After all were drawn with the camera- lucida, no cribed, until the final arrangement of the figures on the 



transfers were permitted, therefore the shells are all plate had to be considered and carefully studied, in order to 



reversed. This defect, however, was considered prefervable show as many of the natural relations of the species and 



to the risk of errors sure to occur in any attempt at trans- series, as possible. 



