100 Proceedings of the Biological Society of Washington. 

 Pteroplatea maclura (LeSueur.) 



BUTTERFLY RAY; SAND SKATE. 



This ray, like the preceding, is very abundant at Beaufort. In 1912, I 

 took 4 female specimens in Newport River, all of about the same size, viz., 

 26 to 27 inches wide and 16 or 17 inches long to tip of ventrals. All had 

 the reproductive organs on both sides functional, but the left ovary was in 

 all cases better developed and the left uterus invariably contained more 

 eggs than the one on the right. In all specimens a shell-gland could be 

 found just anterior to the uterine enlargement. The uteri were on the 

 interior closely crowded with richly vascularized villi, and were generally 

 filled with a milky secretion. 



Careful examination of the uterine eggs revealed some interesting struc- 

 tures'. The left uterus of ray number I contained 3 yellow yolks enveloped 

 in very thin diaphanous coverings but carrying neither blastoderms nor 

 embryos. The middle one of the eggs had over one end only a fragment 

 of egg shell. The most anterior egg had at its front end a lobe about one- 

 third as large as the main yolk mass. Around the isthmus joining these 

 was a mass of delicate yellowish material, probably a piece of shell, and 

 this had possibly caused a constriction of the yolk, although the main 

 portion of the yolk was of itself as large as either of the other eggs. The 

 right uterus contained an empty egg shell larger at one end and having 

 the other somewhat folded and rolled up. 



Ray number II had a left ovary whose volume was about 25 per cent 

 greater than the right. The right uterus contained but one egg, which 

 was enveloped in a delicate transparent shell of light straw color. On the 

 soft light yellow yolk neither blastoderm nor embryo could be found. 

 The left uterus, which was somewhat larger than the right, also contained 

 but one shell, which however enclosed 2 yolks, each the size of that in the 

 right uterus. On one of these yolks was found a very small embryo in 

 the selachian stage. This is the first case of polyembryony (if it may so 

 be called) which I have ever met with in my dissections of sharks and 

 rays. However, such have been previously reported by others. 



Home (1810) found a single egg shell of Squalus acanthias to contain 

 3 yolks. Haacke (1885) has at some length described polyembryony in 

 2 Australian rays, Trygonorhina fasciata and Rhinobatus vincentianus. 

 Redeke (1898) has figured a polyembryonal capsule for Trygon pastinaca. 

 Joseph (1906) examined an egg-shell of Scyllium having in it 2 yolks 

 flattened at the point of contact. However, most remarkable of all is 

 Swenander's (1907) account of finding in the uterus of thecommon nortb- 

 ern mackerel shark, Lamna cornubica, " * * * above 40 pieces of peculiar 

 structure stuck together, which on close examination were found to be 

 egg-masses enclosed in a common shell." While Vayssiere (1909) 2 years 

 later obtained, from the left oviduct of a shark of the same species, a 

 large egg-shell 34 x 92 mm., which on being opened was found to contain 

 2 yolks each having on it an embryo 15-16 mm. long and perfectly normal 

 in all respects. 



The reproductive organs of the third butterfly ray were functional on 



