Close up of Black Rat Snake scales, Bayshore Road, Cape May. Note the keel or line which runs down each scale and mostly dies out before reaching the tip of the scale. In this picture, the front of the snake is to the right. [Photo by Mike Crewe]
Black Racer scales, The Beanery, Cape May. Note the lack of a keel. The scales also seem to be slightly narrower in profile which might be a useful ID feature, though it would need the observer to be very familiar with both species. In this picture the front of the snake is to the left. [Photo by Mike Crewe]
Typically, the books say that Black Rat Snakes have much more extensive white on the belly which comes well up onto the throat and even onto the sides of the mouth. Black Racers have some white but it tends to be less extensive. This seems OK but I hate comparative 'tendencies' like this as you may need to have both species present before you can make a 'more than/less than' judegement call.
Where Don's photo really helped me (as I have not seen a juvenile racer myself yet) is that I can see that there appears to be a clear difference in head shape between the two species which seems to hold at all ages. Though the head may be difficult to see properly if the snake has been disturbed and is moving away, practice and familiarity should make it possible to tell the two apart more easily than by using the scale keels alone. Give it a go, and let's see if it works! It's perhaps worth adding that, in my experience, Black Rat Snakes are overwhelmingly the more common of the two species around Cape May, being found in pretty much any habitat and commonly found around houses and backyards. To date I have only seen two Black Racers, one at The Beanery and one at Higbee's Beach WMA, so it may be that they prefer open fields and woodland edge.
Head of adult Black Racer at The Beanery. Compare with Don's photo of a youngster and notice that, in both snakes, the line of the brow starts to angle down towards the tip of the nose just in front of the eye (where you can see yours truly reflected!). This gives the snake a rather snub-nosed look. [Photo by Mike Crewe]
Head of adult Black Rat Snake, CMBO Northwood Center, Cape May Point. In contrast to the racers, the rat snake's nose seems to be in line with its eye and the brow line continues in a straight line, only dipping down right at the end, beyond the nostrils. Incidentally, note the extensive white coloration, extending well up past the mouth to the bottom of the eye and around the nostril on this individual. [Photo by Mike Crewe]
Juvenile Black Rat Snake, Bayshore Road, Cape May. Notice that the brow line runs straight out to the nose without bending down until past the nostrils, at the very tip of the nose, just like in the adult above. Note also that rat snakes tend to have paler irides than racers, but this may not always be so obvious as here. [Photo by Mike Crewe]
This does seem to be a useful feature and should be useable to identify photos, where the snake is often not close enough for the scales to be checked accurately for keels.
BIG PS: Neither of these species is poisonous!!However, Black Rat Snakes will not hesitate to attack if they feel cornered - as I found out a few years ago at the State Park!