word choice - Types of things vs. types of thing

When speaking precisely or technically, one would say that "Homo erectus and homo sapiens are two species of hominid" rather than "Homo erectus and homo sapiens are two species of hominids." The hominid here should be singular because we are speaking about instances of a single class ("class" being used here in its broader sense, not in the sense of taxonomic grouping).

Now let's consider more common parallel constructions replacing the word species with other words.

Tide and Wisk are two brands of detergent.
Tide and Wisk are two brands of detergents.

Cats and dogs are two types of pet you can buy at Pets-R-Us.
Cats and dogs are two types of pets you can buy at Pets-R-Us.

I feel that using the singular noun to specify the class is more grammatically correct, but sounds stilted in conversation. Almost everyone I hear making these constructions uses the plural form. What are your thoughts about singular vs. plural here?

Edit

Here's a more extreme example that may help clarify the issue. Consider the following sentence:

It was my first attempt at calming an angry crowd, and I just
stood there while people were hurling all kinds of insults at me.

Here I think the singular insult sounds strange, if not downright wrong.

Further Edit

I just noticed in A Treasury for Word Lovers (Morton S. Freeman, 1983) a section entitled "Types of Errors." This is a book about (American) English usage by an English professor and editor, which purports to be a "practical guide for serious writers and readers." I wish I could say this discovery satisfies the question, but in fact I now feel somewhat farther from the truth, if there is any single "truth" in this matter. But at least I don't have to worry about being wrong when using the plural form of the class in these constructions.

Answer

This is an interesting question, particularly because of this dichotomy:

• This is a type of apple. (not apples)


• These are two types of apples. (not apple)

I think that the construction of the form "two types of apple" sounds more than stilted; it just plain sounds awkward, and I would be surprised that it sounds familiar and normal to anyone (at least speaking for US English).

The idea that a plural form would be used for a class is actually not strange at all in English. To express the fact that I like things belonging to the "apple" class, I would say:

• I like apples.

I would not be able to use the singular to express this:

• *I like apple.


• *I like an apple.


• *I like the apple.

Saying "I like apples" doesn't even imply that I am talking about multiple apples; one could say this, for example:

• I like apples, although I've only ever had one in my life.

So, saying "I like all sorts of apples" seems to jibe perfectly with the rest of English grammar.

This means that the strange case is actually this one:

• This is a type of apple. (not apples)

Saying "this is a type of apples" is definitely not natural or familiar. It seems that, in phrases like "type(s) of X" ("kind(s) of X", etc.), there is generally number concord between the type-word and the class itself. Why that is, I don't know.