24 



SHOKTKn C'OXTinisrTloX.S To CiEXERAL (iE(IL()(JY, llt21. 



Porto Kico. (hihaidax cncpa from Cuha, lierc 

 descriyjed as new, coiiiplotos the list of known 

 species of (hiliaiihix. 



CRITERIA FOR DISCRIMINATION OF SPECIES. 



Ill discriniinating between speeies of Orthaii- 

 lax several classes of criteria may he used: 



The external form of the shell is of value but 

 can not be relied on as infallible. Cross sec- 

 tions of 0. inoniatiix, O. a</iiaililh iisix, and O. 

 caepa, so far as we know, are always nearly 

 circular, and (). piigna.r and O. f/ahhi are usually 

 sxibtriangular, but many immature specimens 

 of O. gahhi are circular. Oriliniihix c<i( pn and 

 0. agi/adillensift each have two forms, a. conical 

 and a hemispherical, which possibly are sec- 

 ondary sexual characteristics. There may be 

 a dome-shafx'd or hemispherical form of 0. 

 inoniaiiis also, but it has not been discovered. 

 The conical forms of (K inornatiit^, O. <■<!( pa,And 

 0. agwidillni.'iis may be discriminated from one 

 another by the apical angle, which is most 

 acute in (). inoniatiis and most obtuse in 0. 

 aguadiUciinh. 



Of considci-able value in discriminating be- 

 tween species is the east of the interior. The 

 casts show the altitude of the whorls and their 

 rotundity or llatness, features which ar(> con- 

 cealed l)y the shell, and the outlines of the 

 living chamber, which usually differ consider- 

 ably from the external form. 



Trustworthy identihcations of some speci- 

 mens can not be made without cutting. Axial 

 sections tiirougb the apex disclose ail the es- 

 sential features of form and structure. The 

 onion-lik(^ arrangement of the layers of shell and 

 callus is laid bar(>, the outlines, both internal 

 and external, of the wiiorls at all stages of 

 growth can be traced, and the interrelations of 

 shell substance, enamel, and cavities are plainly 

 visible. 



OCCURRENCE AND STRATIGRAPfflC POSITION 

 OF SPECIES. 



Orthaulax iiioniaiun. — Ort/i a iilax i n o r not u s 

 Gabbjthe ty])e of the genus, was hrst discovered 

 in Santo Domingo. Specimens of this species 

 were collected by Col. T. S. Heneken in 1848 or 

 1849, but they lay for many years luidescribed 

 in the collection of the Geological Society of 

 London. The locality at which they were 

 found is not known. One of Heneken 's Domin- 

 ican specimens was figured by Guppy in 1876, 



and the figure is reproduced here (PI. II, lig. .5). 

 Gabb figured specimens of Ortluiulax inornatus 

 in 1872 fi-om Santo Domingo, but as he was 

 addicted to the labor-saving but highly repre- 

 hensible habit of stuffing his pockets with un- 

 labeled specimens, the locality at which he 

 found them is not recorded. 



The stratigraphic position occupied by Or- 

 thaulax hioniatiis in Santo Domingo long re- 

 mained unknown. Because of the presence of 

 this species somewhere in Santo Domingo, pre- 

 sumably in the valley of Rio Yaque del Norte, 

 Maury'' postulated an "Orthaulax inornatus 

 formation" regarding which she says: 



No one knows where Gabb found his Orllmiilax momalus 

 because none of his r)oniinican fossils were lalieled more 

 precisely than "Miocene, Santo Domingo. ' ' We presume 

 however, that it was collected from the basal blue clays 

 in the eastern part of the Yacjui Valley, where the revolu- 

 tion pre^-ented our securing sections and collections. For 

 in the western part of the valley we have gone almost to 

 the contact of the Tertiary with the older series, and 

 although a special search was made for Orlhaulnx we did 

 not find it. No doubt the Orthaulax -/.one represents an 

 older horizon than [any] from which we ccllected. 



Regarding the correlation of this hypotheti- 

 cal "Orthaulax formation" she says- * 



The Dominican Orthaulax formation is older than those 

 of Bordeaux and represents approximately the Rupelian 

 Oligocene of Europe and the Tampa silex beds and White 

 Beach limestone of Florida. It comes in just above the 

 Vickslnirg limestone. 



The expedition to the Dominican Republic 

 in 1919 headed by T. W. Vauglian was so fortu- 

 nate as to pi'ocure a typical specimen of Ortliini- 

 lax inomatiis associated with so chai'acteristic a 

 fauna and in beds occupying so luimistakable a 

 stratigraphic position as to dispel all <loid)t as 

 to its age. The Orthaulax was collected on Rio 

 Yaque del Norte at Baitoa from the lower part 

 of the Baitoa formation (basal Miocene), which 

 lies unconformably upon the tilted conglom- 

 erates and shales of the Tabera formation 

 (middle Oligocene). The corals and mollusks 

 with which the Orthaulax is associated have 

 been studied by Vaughan and Woodring, who 

 correlate them with the Burdigalian fauna of 

 Eiu'ope and with the fauna of the Chipola 

 marl member of the Alum Blufl' formation of 

 Florida.'^ 



3 Maury, C. J., Hull. .\in. I*ah'onfnlogy, vol. .'., jij). 4.'ttV-4r.7, l'JI7. 



' Idt-m, p. 4.^, 



'' \'aughan, T. W., and Woodring, W. P., Tertiary and C^uutcrnary 

 stratigraphic puleuntology: A geological reconnaissance of th? i'omiai- 

 can Republic, p. W>, 1921. 



