14 DE. J. F. GEMMiLL OX [May 12, 



off (1) a hyoid artery which sends a branch to the correspondmg 

 outer pseudobranch, and (2) a carotid artery. It then joins the 

 second aortic root, and shortly afterwards meets the continuation 

 of the ventral aorta previously mentioned. The resulting vessel 

 is then joined by the third and fourth aortic roots, but it remains 

 separate from its fellow so long as the notochords are separate, 

 i. e. back to the twentieth somite. 



The arrangement of vessels which has just been described and 

 which is illustrated in PL II. fig. 13 is somewhat remarkable. 

 Mixing of arterial with venous blood must have taken place in no 

 slight degree, and certain parts in each twin head must haA^e been 

 supplied by blood coming directly from the ventral aorta. There 

 is no trace of a pair of adjacent jugular veins, and it is difficult 

 to make out from sections the course of the venous blood coming 

 from the adjacent sides of the twin heads. But as there is a con- 

 siderable amount of spongy tissue below the base of the skull and 

 in the septum between the mouth-openings, it is probable that 

 the blood in question found its way into the median and the main 

 jugulars. The presence of this spongy tissue no doubt indicates 

 congestion. 



The pronephric glomerulus (PI. IV. fig. 32) is composite. It is 

 remarkably large, and is divided into three compartments by two 

 vascular tufts each of which has an afferent and an efferent 

 vessel. Normal Wolffian ducts arise from the outer compart- 

 ments, while the middle compartment gives origin to a sacculated 

 tubule which passes backwards a short distance to end blindly, 

 and which represents fused adjacent Wolffian ducts. 



There are two buccal cavities, but the oesophagus and the rest 

 of the alimentary tract are single except for the presence of two 

 air-bladder diverticula. 



The composite muscles described on page 9 are well developed 

 in this monstrosity. 



II. Union in Pectoral Region. 



Class II. of the monstrosities has next to be described. In it 

 the twin bodies are united in the region of the pectoral fins. All 

 specimens of this Class have separate gullets, air-bladders, and 

 stomachs, but a single liver, intestinal canal, and vent. The 

 hearts are more or less closely united. Two subgroups of this 

 Class are naturally distinguished from one another — {a) a group 

 in which union of the twin bodies takes place so far forward that 

 adjacent pectoral fins are not present ; and (6) a group in which 

 adjacent pectoral fins are present, but a,re united and reduced in 

 size. 



(a) Adjacent Pectoral Fins not represented. — The brain, the 

 cranial and branchial skeletons are double. So also are the 

 alimentary canals down to the level of the stomach. The chief 

 interest in this type centres round the heart and blood-vessels. 

 A reconstruction drawing of the heart and origins of the vessels of 



