Contrary to what is observed with the venoms of VIPERIDÆ in general, all the venoms of COLUBRIDÆ and, as exceptions to the rule, the venoms of some North American CROTALINÆ (_Ancistrodon contortrix_ and _A. piscivorus_) suppress the coagulability of the blood _in vivo_ and _in vitro_. It is, however, important to observe that, _in vivo_, the blood remains fluid after death only if the dose of venom absorbed has been sufficient. _In vitro_ this phenomenon is easier to study, and has been the subject of several important memoirs. Halford,[39] Sir Joseph Fayrer,[40] C. J. Martin,[41] Delezenne,[42] Phisalix,[43] and lastly Noc,[44] have shown that the venoms of COLUBRIDÆ exert a manifestly anticoagulant action upon citrate-, chloridate-, or oxalate-plasmas, and also upon blood mixed with venom on issuing from the vessels. On adding 1 milligramme of _Cobra_-, _Bungarus_-, Australian _Pseudechis_-, or _Ancistrodon_-venom to 1 c.c. of citrate-, oxalate-, or chloridate-plasma, and supplementing the mixture, after varying periods of contact, with a quantity of chloride of calcium (for the citrate- or oxalate-plasmas), or distilled water (for the saline plasma) sufficient to produce coagulation in a few minutes in the control tubes without venom, we find that coagulation no longer takes place after one hour in the tubes containing _Cobra_- or _Bungarus_-venom, and after ten minutes in those that contain the venom of _Ancistrodon_. In doses less than 1 milligramme for 1 c.c. of plasma, these venoms by themselves never produce coagulation as do those of _Lachesis_ or _Vipera russellii_. They are thus sharply differentiated in this respect. If fresh blood issuing from the arteries of an animal be received in a vessel containing a sufficient quantity of COLUBRINE-venom (that of the Cobra for example), and steps be immediately taken to ensure the perfect mixture of the venom and the blood, we find that the latter has entirely lost its coagulability, just as though it had been mixed with peptone or extract of leeches’ heads. Again, if a mixture be made _in vitro_ of coagulant venoms, such as that of the _Lachesis_, with anticoagulant venoms such as that of the _Cobra_ or of _Ancistrodon_, it is found that these mixtures, when properly effected, become neutral, so that the respective effects of the component venoms are entirely destroyed. Assuming, for instance, that 1 milligramme of _Lachesis_-venom coagulates in two minutes 1 c.c. of 1 per cent. citrate rabbit-plasma, if we add to the plasma firstly 1 milligramme of _Ancistrodon_-, or 1 milligramme of _Cobra_-venom, and then 1 milligramme of _Lachesis_-venom, the plasma remains non-coagulated, yet coagulates perfectly on the subsequent addition of 1 c.c. of a ½ per cent. solution of chloride of calcium.