88 BULLETIN 71, UNITED STATES NATIONAL MUSEUM. 



PENEROPLIS PERTUSUS (Forskal), var. ARIETINUS (Batsch). 

 Plate 36, fig. 2; plate 37, fig. 5. 



Nautilus (Lituus) arietinus (part) Batsch, Conch. Seesandes, 1791, p. 4, pi. 6, 



fig. 15c. 

 Peneroplis arietinus Parker, Jones, and II. B. Brady, Ann. Mag. Nat. Hist., 



ser. 3, vol. 16, 1865, p. 26, pi. 1, fig. 18.— H. B. Brady, Rep. Voy. Challenger, 



Zoology, vol. 9, 1884, p. 204, pi. 13, figs. 18, 19, 22. 



Description. — Test planospiral in its early stages, later becoming 

 uncoiled and building chambers in a linear series, in transverse sec- 

 tion elongate elliptical, wall longitudinally striate, aperture consist- 

 ing of an irregular series of pores in the middle line of the apertural 

 face. 



Distribution. — Rhumbler has recorded this variety from shore 

 sands of Laysan Island. I have it from Nero stations 2039 in 24 

 fathoms and 2042 in 55 fathoms off the Hawaiian Islands, from Gas- 

 par Straits, and Tuscarora material from Manila Bay. 



Genus ORBICULINA Lamarck, 1816. 



Nautilus (part) Fichtel and Moll (type, 0. adunca (Fichtel and Moll)), Test. 



Micr., 1803, p. 112. 

 Orbiculina Lamarck, Ency. Method., pt. 23, 1816, p. 468. — H. B. Brady, Rep. 



Voy. Challenger, Zoology, vol. 9, 1884, p. 208. 



Description. — Test planospiral, at least in its early stages, the 

 chambers numerous, and in the later stages, at least, subdivided into 

 chamberlets, the early chambers in all forms extending over the 

 early test to the umbilical region, making a completely involute test 

 in the early stages, later tests may continue the completely involute 

 character, or may become annuli or build a crosier-shaped test, wall 

 usually pitted, sometimes smooth ; aperture in the adult usually con- 

 sisting of a double row of small circular openings usually opposite, 

 along the median portion of the apertural face of the test. 



This genus may most easily be distinguished from Peneroplis by its 

 subdivided chambers and from Orbitolites by the early chambers 

 which in Orbiculina are involute, causing a thickening of the test in 

 the umbilical region. From Orbitolites duplex it may be distinguished 

 by the pores which in circular specimens of Orbiculina are near the 

 median line of the apertural face and usually opposite, while in 

 Orbitolites duplex they are near the peripheral margins and usually 

 alternate. 



Orbiculina is not at all common in the North Pacific, but from the 

 records appears to be common about the East Indies and Philippines. 

 A comparison of the development of the two most distinctive forms 

 0. compressa and O. adunca is here given for a comparison on one hand 

 with Peneroplis and on the other with Orbitolites. 



